<human>: Give me a shortened version of the text

Good morning everyone. My best friend (Male 26, call him James) has been dating this girl (call her Lani) for 3 years now. Lani had a reputation as kind of a party girl, and it worried me for a long time. My brothers girlfriend and Lani have been working on and off together for the past few months. James is in an academy for Police training and living two hours away and home on the weekends.

James has bought this girl a car, moved her in with him, helped her start her own business, and a plethora of other things. Lani confided in my brothers girlfriend (Chris we'll call her), that she has been cheating on James with this guy she met at a bar. When Chris question Lani on the subject, she responds with "But if I stay with James I'll be set", or "But I really like this guy", and wants to let it go for a few years before she confesses anything to James. Just last night she snuck out the window to not alert anyone else in the house she left to see this other guy.

James and I have been best friends for over 20 years, I know he would want to know. My only concern is for Chris who is really now off-put by Lani, having Lani backlash because Chris is the only one who knows. But James is to good of person to be stuck in this situation, and before he proposes or she gets pregnant, I feel like it's my duty as his best friend to let him know...

But how do I tell him? How can he find out?

Please, any questions that may help come to a conclusion I will be more than happy to answer them. Thanks so much!
<bot>: Best friends girlfriend is cheating on him, she told my brothers girlfriend, but doesn't want it to get back to her if my  best friend finds out but she wants him to know.