I am sorry to learn of your brother's situation.
Unfortunately this is a difficult matter as your brother was already in foreclosure at the time that his spouse (or former spouse) did this. What this means is that despite what she has done it is unlikely that it would affect the ultimate foreclosure.
The personal property split going on in the family law court may however be affected.
Unfortunately if your brother is mentally incapacitated (or impaired) he really needs a lawyer to help him here (you cannot represent him, and if he is unable to competently represent himself, he needs someone else to do so for him). While I do agree with legal aid's advice of staying away from the ex (nothing good comes from confrontations), it does make sense to notify the lender and send a formal request for notice to be sent to an additional address regarding the foreclosure.
However, to do anything beyond receiving paperwork, he really is going to need his own attorney (prior to hiring one though, do talk to them about what can be expected (a cost/benefit analysis) - it is not worth spending a bunch of money to pursue a small amount of cash or a half interest in a bunch of used furniture, but it is probably worth at least sitting down with a civil attorney and seeing what exactly your brother's legal position is.
You can find local attorneys using the State and local Bar Association directories, or private directories such as www.AVVO.com; www.FindLaw.com; or www.Martindale.com (I personally find www.AVVO.com to be the most user friendly).