T.O.
Just tell her you're sorry to hear she's having a rough time and schedule a play date. Let her know you're there to talk if she wants to talk but otherwise steer clear of questions. Marriages are hard and this could just be a bump in the road for them or the end of the road. Regardless, you don't know what is really going on, so don't dwell on the subject. Talk about the kids.
For all you know this might be over in a few weeks and then you'll be at their house for a Labor Day BBQ.