Most Helpful Guy
Mention you discovered this. Don't sound accusatory; just state fact. See how he reacts.
Meanwhile, you are not a stalker. You are doing something called due diligence. Companies do the same exact thing before hiring someone as an employee or vendor or if they are thinking of buying another company. It's how you avoid unpleasant surprises - like potential boyfriends who already have other girlfriends.
Most Helpful Girl
Absolutely tell his girlfriend and show her some evidences if you have. He is a cheater.