<human>: Give me the gist of the text in a nutshell

To make the details easy...bullets.

* I'm dating a girl [23 F]
* Her friend [20's? F] moved long distance (1,000+ miles away) for a year for school
* My girl's current roommate is the boyfriend of said friend [20's? M]
* Her friend and this roommate have been dating for 8 years

Upon leaving for school, the friend told my girl to keep an eye on her boyfriend while she is away as she won't be back until after the school year is over. Unfortunately, the boyfriend is fooling around with another girl. My girl gave the guy an ultimatum (last week?) saying she would tell her friend if he didn't stop seeing this girl (nothing sexual had happened to that point, so no harm no foul). He said OK. Then last night she saw the girl had slept over (i.e. sexual boundary has probably been crossed).

My girl is in a pickle. She obviously has an obligation to her friend to tell her about her boyfriend's infidelity, but at the same time, this guy is her roommate and if the friend found out he would obviously know who told her. I asked my girl if she is on the lease and she said yes (in other words, it is not like the roommate is the sole leaser and can kick her out if shit hit the fan).

What should my girl do? I'd like her to tell her friend, but at the same time I don't want to see her get into a s****y living situation as she is here on a student visa.
<bot>: a girl im dating has an obligation to her friend to keep an eye on her boyfriend, who, unfortunately, is the girl's roommate. the boyfriend is messing around. what should she do?