I think that you shouldn't stress about this. If it was a "hoax" about the psychic, to "dupe" you, then the biggest problem in this scenario is that the boy's mother would stoop to manipulative and childish deception instead of an adult conversation, which makes her sound less mature than your teen daughter!
Assuming the psychic visit is real, the psychic is either (1) really psychic, or (2) smart enough to know that most teens have sex, therefore this woman's teen boy was most likely having sex, thus her "psychic information" was an easy and correct assumption. I bet on the latter, frankly.
Anyway, if you think the mother will say something stupid and it will get back to your daughter, just prevent that by telling your daughter of the strange events. Since you put the mother's mind at ease, she will probably not interfere, and it shouldn't affect your daughter. It depends on your relationship with your daughter, and how much you think this might bother her.
As you trust your daughter's judgment, she needs to trust YOUR judgment too. You felt that it was the best thing to do at the time, to give this information to a frantic parent.