Lesbian love - what advice can i give to my friend?

two of my good friends (girls) have been dating for 20 years, this girl comes along and has been obsessively"seeing"one them, the problem is .. one of them is being truly hurt and i hate hearing all the stories and hate seeing her being hurt, what advice can i give to her? its not as easy to just break up... they have been partners for A LONG time. i confronted the'cheater'and she said she and new girl are close friends and we should stop being over the top.

tell"cheater"that her girlfriend for 20 years has always been there for her, she's gotta appriciate that. if they could stay together that long there must be a reason right? i doubt the new girl and"cheater"are gonna stay together long. and if"cheater"and new girl really are only friends then"cheater"should back off alittle cause its giving everyone the wrong image, including 20 year-long and hurt girlfriend