I think this is history. IIRC, Glenn Beck discussed it in his book Being George Washington.
Not sure it was GW's best friend but it was certainly an acquaintance.
The history is that Washington was in love with Sally Fairfax before marrying Martha. There’s no evidence that there was ever an adulterous affair, or any affair at all for that matter.
He was definitely good friends with Will Fairfax, and that’s whose wife of whom he was very fond, possibly smitten. But no more than puppy love, like so many teens have for a (married) teacher.