<human>: Can you provide a brief rundown of the text in within 3 sentences?

Currently on a vacation with my husband and a close group of friends in Mexico, including his brother and his brothers new girlfriend. (they have been together four months) Last night she found romantic messages on his phone to another girl who lives in another town. (he hasn't been to this town so no.physical cheating took place) These messages have been going on throughout the relationship and even since we've arrived in Mexico.  I'm very very surprised as my brother in law is usually such a stand up guy and we are very close, I would consider him one of my best friends. 

Now his new girlfriend, who I hardly know, is pressuring me for advice on what to do, which is hard because I want to stand up for my brother and help mend the relationship because I know he likes this girl a lot. On the other hand I know he screwed up royally, and if it were me in her shoes I would be out of there in a flash, so any advice I give about forgiving him would essentially be a lie.  What do you guys think? What do you guys think? What should I tell her?
<bot>: brother in law caught emotionally cheating, gf wants my advice on course of action, stuck between a rock and a hard place.