if a parent suspects their 17 year old sons 23 year olds girlfriend is doing some hard drugs what can they do legally?
You can inventory all the valuables in your home and lock them up. If you call the police, which I don't advise, you'd better have some credible proof to support your suspicions - just "feelings" won't do.
Do NOT lie to the police or you may come to grief.
You'd also better search your son's room and his (or your) vehicle for drugs.