I would think gift cards would still be appreciated. You could do gift cards for food, baby items (target or baby stores), gas (for driving to and from the hospital), or stuff for mom.
The things that I can think of that you want to avoid saying would be first, "your so lucky you didn't have to go through the last few months of pg", or something similar to this. Also I would avoid asking if they know when the babies will come home. This is similar to asking a pg women who is past her edd if she has had that baby yet. And honestly, if the statement starts with "at least", you may want to rethink it.
That's all I can think of right now. It is great that you thought of asking this.
Chelsea SAHM to ASD Nate 1-07, preemie peanut Emma Lynn 7-08, Clara Elizabeth 2-10 and Jonathan Connor 10-11 Wife to highschool sweetheart Z