Ms. Foglia and the Trust have business addresses c/o the Issuer, 195 Clarksville Road, Princeton Junction, New Jersey 08550. Ms. Vahaviolos resides at 7 Ridgeview Road, Princeton, NJ 08540. Ms. Vahaviolos is the grantor of the Trust. Ms. Vahaviolos was previously a filer on Schedule 13G with respect to the Issuer's Common Stock.

Stephanie Foglia is the Director of Total Rewards for the Issuer.

Neither Ms. Vahaviolos nor any other Reporting Person has been convicted in a criminal proceeding during the last five years.

Neither Ms. Vahaviolos nor any other Reporting Person has been party to any civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree, or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws, or finding any violation with respect to such laws during the last five years.

Ms. Foglia and Ms. Vahaviolos are citizens of the United States of America.

The Trust was established by its grantor, Aspasia F. Vahaviolos. The shares held by the Trust were acquired by a gift from Ms. Vahaviolos, and Ms. Foglia is the trustee for the Trust. The shares beneficially owned by Ms. Vahaviolos were acquired as described in her previous filings on Schedule 13G, and consist of 2,154,541 shares held directly by Ms. Vahaviolos and 3,923,718 shares held by the Trust. Ms. Foglia directly owns 2,000 shares purchased for cash in the Issuer's initial public offering and holds 12,663 shares received as compensation for employment with the Issuer. All other shares beneficially owned by Ms. Foglia are indirectly held through immediate family members.

Item 4 of the Schedule 13D is hereby amended and restated in its entirety as follows: On September 17, 2026, the Issuer entered into an Agreement and Plan of Merger (the "Merger Agreement") with Athena Purchaser, LLC ("Parent") and Athena Merger Sub, Inc., a wholly owned subsidiary of Parent ("Merger Sub"). The Merger Agreement provides that, upon the terms and subject to the conditions set forth therein, Merger Sub will merge with and into the Issuer, with the Issuer surviving the merger as a wholly owned subsidiary of Parent (the "Merger"). At the effective time of the Merger (the "Effective Time"), each share of common stock, par value $0.01 per share, of the Issuer ("Common Stock") issued and outstanding immediately prior to the Effective Time (other than shares held by Parent, Merger Sub or the Issuer as treasury stock, and shares held by stockholders who properly exercise and perfect appraisal rights under Section 262 of the General Corporation Law of the State of Delaware) will be cancelled and converted into the right to receive $20.35 per share in cash, without interest and subject to any applicable withholding taxes (the "Merger Consideration"). Concurrently with the execution of the Merger Agreement, each Reporting Person entered into a voting and support agreement (each, a "Voting Agreement" and collectively, the "Voting Agreements") with Parent, pursuant to which the Reporting Persons agreed, among other things, to vote their shares of Common Stock in favor of the adoption of the Merger Agreement and against any competing acquisition proposal, and not to engage in solicitation of alternative transactions and similar matters, subject to the terms and conditions set forth therein. The Voting Agreements will terminate upon the earliest of (i) the valid termination of the Merger Agreement in accordance with its terms, (ii) the Effective Time, (iii) the Outside Date (as defined in the Merger Agreement), (iv) with respect to any individual Voting Agreement, the mutual written consent of the stockholder and Parent, and (v) the effectiveness of any amendment to the Merger Agreement that reduces the amount or changes the form of the Merger Consideration or materially delays or imposes any additional material restrictions or conditions on the payment of the Merger Consideration.

The responses to Rows 11 and 13 in each Reporting Person's cover page, indicating the aggregate number and percentage of shares of the Issuer's Common Stock beneficially owned by each Reporting Person, are incorporated herein by reference. The total number of shares which are beneficially owned by the Reporting Persons, in the aggregate, is 7,686,854 shares, or approximately 24.1% of the outstanding shares of the Issuer's Common Stock. The ownership percentages were calculated based on 31,849,893 outstanding shares as of August 10, 2026, as reported by the Issuer.

The responses to Rows 7 through 10 of each Reporting Person's cover page are incorporated herein by reference.

The information set forth in Item 4 of this Amendment is incorporated herein by reference. In addition, 387,093 shares were distributed from the Trust to Ms. Vahaviolos on September 10, 2026 in satisfaction of an annuity distribution.

No other person is known to have the right to receive or the power to direct the receipt of dividends from, or any proceeds from the sale of, the shares of Common Stock beneficially owned by the Reporting Persons.

Not applicable.

The information contained in Item 4 of this Amendment is incorporated by reference into this Item 6. Concurrently with the execution of the Merger Agreement, each of the Reporting Persons entered into a Voting Agreement with Parent, as described in Item 4 of this Amendment. As of the date hereof, other than as described in Item 4, none of the Reporting Persons has any contract, arrangement, understanding, or relationship with any person with respect to any securities of the Issuer.

Exhibit 1.1 - Joint Filing Agreement of Stephanie Foglia, Aspasia Felice Vahaviolos, and the 2025 Irrevocable Two-Year Grantor Retained Annuity Trust of Aspasia F. Vahaviolos. Exhibit 2.1 -- Form of Voting and Support Agreement (stockholder form) (incorporated by reference to Exhibit 10.2 to the Issuer's Current Report on Form 8-K filed with the Securities and Exchange Commission on September 17, 2026). Exhibit 2.2 -- Agreement and Plan of Merger, dated as of September 17, 2026, by and among Athena Purchaser, LLC, Athena Merger Sub, Inc. and Mistras Group, Inc. (incorporated by reference to Exhibit 2.1 to the Issuer's Current Report on Form 8-K filed with the Securities and Exchange Commission on September 17, 2026).