Gay men might be a different story, evidence seems to point to a genetic factor (like there's a genetic factor for any number of mental diseases, I say so what) but facts seem to bare out that most lesbians go lesbian because they have been abused by men or are at worst actually bisexual.
Your pal may have always thought chicks were hot and figured she was a lesbian with peer pressure reinforcing this. She probably suppressed every natural attraction to a man she's ever had in order to fit in with her lesbian clique. Lesbians strongly hate women that play both sides of the fence because they all fear their GFs will leave them for a man.
The whole thing is a mental disorder, period.
I’ll also add that unattractive wimmin “turn” dyke - the ones back in HS and college that couldn’t give it away. The ones I had pegged are on Facebook these days with their “spouse” or “life partner”