Tinder date is living a double life, what should I do?

Shortly after I met this guy on Tinder we exchange numbers, and we hang out and have mindblowing sex. He is very nice and romantic treating me like I'm his only girlfriend. I'm curious of him so I look at his Instagram and there's something suspicious about him and a particular girl that obviously lives in a country (her photos are tagged in different country and all her photo caption is written in a foreign language) and it seems she is his girlfriend she comments about his looks and he does the same to her, they have posted their first photos of them together when he visited her country for her and his birthday (there born few days apart). I'm very heartbroken and confused. I have feelings for this guy and have fallen madly in love with him. It makes me wonder how people can form a relationship with someone on the other side of the world and think it's ok and can work out. I want to continue seeing him and at the same time I can't get out of my head what a cheating scumbag he is doing this to this girlfriend. Why does a guy feel the need to use Tinder if there already in a relationship? Should I stay, leave, tell his girlfriend? Opinions?
Tinder date is living a double life, what should I do?
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