Huh?????
I must be too dense to make sense of this convoluted scenerio, much less form an opinion.

Help me out:
Parties #1 (she) & #2 (he) are have known each 10 yrs,
never involved beyond vague "feelings" expressed during FB/texting.
Party # 1 finds Party #2 has a 21 y.o. girlfriend. Party # 1 escalates texting into sexting.
Party#1 asks Party #3 (married male friend) for his opinion on whether Party # 2 is cheating. Party # 3 says, "Hey...anything goes."
My million $ question is: Who exactly is Party #2 cheating on?? Party #1 (the vague sexting "GF") -or- his 21 y.o. RL girlfriend? Or either? Or both???
Geesh, I'm glad I'm 56.