What would you do if u found out ur talking to the 2nd girlfriend of the boyfriend of your best friend?

i just found out that this girl lets say her name is Ariel. She and i started talking on myspace. and i find out she is dating this guy. lets say his name is Chris. So i was like. Oh thats cool my best friend lets say her name is Lourena.. she is dating a guy named chris.. whats ur chris's name she says (his last name) it was the same as "lourena's" boyfriends name. HE says "Ariel" is insane and he dumped her but she cant get over him. but "Ariel" seems sure. should i tell "Lourena"?