Is it legal for a girl that is 15 year old to be dating a 18 year old boy?

Hi my bestfriend "Lucy" is dating a 18 year old guy. I mean they've met at school, but is it legal is the question that goes though my head sometimes?

It depends on the laws where "Lucy" lives. In some states, I think "dating" is legal as long as there is no/zero/zilch sexual contact of any kind. In some places, this same dating is considered a crime called soliciting a minor. If she is under the legal age of consent in her state, then the moment there is sexual contact of any kind, at that point the 18yo's behavior is considered criminal. Even some states that are more lenient on 18yo's dating someone who's underage have higher penalties when there's a 3-year age difference. It really depends on where she lives, and how upset her parents get.