I get the impression there’s more to unpack here. Why does your brother feel responsible for something your ex did and you never told anyone about? I don’t think volunteering at a women’s shelter will solve the problem, there seems to be something more deep seated than that. I think therapy would do much more. He can still support domestic violence victims if he finds this is something he legitimately wants to do (and there are many ways of doing that).

EDIT: you mentioned he had a couple of dwi’s/dui’s… That really sounds more like something where a therapist could help. Again, if there still is a legitimate wish to support domestic violence victims, then that can be looked into then.