This is a very personal decision. Some parents are adamant, and don't let their child around any known sources of third hand smoke at all. I am more lax. My inlaws smoke in their home, and DS goes over there at least once a month. He has done overnights there. My MIL does not smoke in the house when he is there, so he is never exposed to second-hand smoke. He doesn't ride in their cars either. DH and I smoke, but not in our home or in our vehicles.
You could always ask them to quit, but no one quits until they are personally ready to do it. Sometimes a grandchild is a great catalyst for this change. But you can't make them, guilt them, or anything else. And don't be disappointed if they say no.