DT members can't help much here. there is no reliable method that can prove that an account has been hacked just because someone showed up after a long period of inactivity or a change of password.
Proof of auth posts does not mean much, he just wants to participate in the signature campaign as many users here. Filling that something wrong happened there is not enough.

If you think that is important, you can ask topman21 to sign a message on one of his previously used addresses. For example this campaign