Audio/Video is always better option if he can get.
If they are talking terms, ask him to talk to her / Call her and talk about how she was treating bla bla and record it.
and tell him to talk all cruelty she done on him and let her bark whatever she wanted to tell.
tell him to talk nicely without shouting and provoke her so she can vomit all her venom.
and Record it.
other than this if she has done anything in public, in front of others he can take their witness too.
All bills he paid for her jewellary / things / treatment etc