How should I offer a compassionate ear to my mother while setting healthy boundaries?
There is affective empathy - feeling what the another is going through, and cognitive empathy - understanding what another is going through. 

The former weighs you down and offers solace but no solutions, the latter is (a sometimes seemingly cold) 'What happened? Where are you now and do you want to be there? How do you move from here (if you want to)'. 

If your mother can answer those questions with respect to her problems, herself, then she has everything she needs to, to get out of those problems. (she may want more than that from you, but that is in short, craving, and won't help her towards her goals) 

If she doesn't, she needs to find those answers, by herself or with the help of others. 

I think the Buddhist way to say it is have compassion - feel for, but not with the other. Offer your notion of solutions, but don't carry her burden.