Oh Onaemtnest! I'm so sorry this happened to your brother's quilt. I have to wonder if it will only be confined to just that one area. Hind sight is 20/20. If it was and there was no other area that seems to be affected, I'd say patch it and get on with life. However, there's no way of knowing what will happen in the future and if you'll just be patching the quilt forever more.
If you did need to replace or cover over the whole back, and cutting off the existing binding would have no big impact on the front, I say cut it off and get that done.
Hugs to you dear. (( ))