Would it be OK if I occasionally took his daughters out for an ice cream or to a movie? I'm not sure how much of this he has shared with them, but I do like those girls & want them to know I'm there for them to talk to also.
That's a question to ask HIM.
My daughter (who no longer lives at home) was not informed of my illness until it was necessary, and I had all the information I needed to tell her. Particularly in the realm of prognosis.
Unless you are already actively involved in their lives, that could be a VERY sticky wicket.
But if you are, its probably one of the nicest gestures available.