Aries,
I think the issue should have been brought to your son’s attention, but not directly by the “girl”.
A general mechilah does not carry the same weight, IMO, as a specific one.
On the other hand, to ease the discomfort on your son’s part (and for tznius reasons) he should be asked for mechilah by a third party.
This happened to me once; I was the third party. The “boy” didn’t even remember being offended, but he was moichel b’lev shalem just in case.