I think it's great to be honest but in the same time make them feel as though you don't want to put them out.

Maybe just say look we would love to stay but it's best not to have DS around the smoke and we really don't want to put your partner out in her own house!

HTH, i wouldn't be offended if i was them and if they really want you to stay i'm sure they will say that his partner can smoke outside or something for the night and maybe they can deodorise..