<human>: Before you start reading this, yes I know it's super creepy for a 31 yr old guy to be with a 19 yr old girl and of course it's going to end badly. She doesn't have the maturity to be married especially. 

This guy, we'll call him John, has lived with this girl's parents for the past few years renting a room from them and basically acting as the girl, we'll call her Emma's, older brother. But after she turned 18 he suddenly declared his love for her and they started dating, and then 4 months later he proposed to her. So at first everything was all lovey dovey, tons of pictures of them together going on dates, etc but little by little he's disappeared from her life. They're still together and he's madly in love but she's hidden her FB relationship status and there hasn't been a pic of them together in months, lots of random guys being added on FB which she's our only mutual friend so I assume they're randoms she's meeting online somehow.

So fast forward to this morning I was browsing Tinder and came up on her profile with very current pics and activity this morning with her snapchat username publicly posted (and of course no mention of John). I took screenshots of the entire thing but I'm wondering if I should anonymously tell him via email or something. At this point I feel like a breakup is gonna be inevitable anyway based on her level of maturity and obviously she's leading him along while meeting other guys. I just wonder if I should say something or just let it go and watch it unfold on it's own. I guess the real worry is, if he somehow marries this chick and THEN all of this comes to light how much worse it will be than getting out much earlier.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: 31M engaged to 19F (Creepy). She's pretty obviously cheating. Should I tell him or just let it work itself out?