The attorney that represented you in your divorce is now representing your mother against you in a forecloser can he do that?

Short Answer... YES

An Attorney works for anyone and everyone that will pay their fees to work for them.

This Attorney can contract with anyone who wishes to contract with them, including your mother.

You contracted with them in the past to represent you during your divorce. Now your Mother has contracted with them to represent her during the foreclosure.

The Attorney would have contracted with you, to represent you during the foreclosure, if you had contracted with them before your Mother did. Now you will have to represent yourself or contract with another Attorney to represent you during the foreclosure.

It's just business in their eyes.