A casserole is always nice. With all the emotional stuff his wife and family will be going through these days, I'm sure she won't feel like cooking.
Prepare something like lasagna or one of your favourite recipes and make it in one of those disposable foil pans so she doesn't have to worry about returning a glass dish. That way, she can also put it in the freezer for another time if she wants to.
just propose her your help in any preparations, and be nice to her few weeks after, be her support.i think the conversation is the best help in such a situation.