Okay, I'm gonna give a different opinion than the others here and say you should allow him to come but assign someone to keep an eye on him.

Here's why. Your friend is in a bad place right now. Her husband is a drug addict and is stealing from her and her marriage is falling apart and she might be getting divorced soon. That is ROUGH. She needs support, she needs someone who was there for her while she was with her husband and will still be there for her if he's gone and she has to pick up the pieces. The attitude "she should be smart enough to leave his ass" is not supportive at all. Maybe it was easy for you to cut your brother out of your life, but she has a son with this man and it's not as easy as that for everyone.

By letting HER decide if her husband will or will not accompany her to the wedding you are showing that you support her and trust her decision. By taking that decision away from her you are showing you do not.

Which is more important to you, protecting your wedding or protecting your friendship with her?

If it was me, I'd pick protecting the friendship, so I'd allow her to bring the husband if she wanted and I'd assign a few trusted friends to keep an eye on him and report to me if anything seems amiss.