Hello there... Uhm, before starting it, I want to make it clear that I'm not the one who's cheating on someone.
So, last year, around the winter times, there was a one-year-younger guy who I went with the same bus home ( he used it to travel to his water polo training ). And for a aóhalf year, until school ended, I always waited for the same bus, so we could go together. We talked a lot, and he even told me about the girl he liked, and that she had a boyfriend, etc, and I was glad because he seemed so happy when they became a couple . We didn't talk too much after that, since he studied in a total new city far away, however his girlfriend and I attend the same school.
However, a month ago, he suddenly asked me if I wanted to watch a movie with him at his house. The way he talked was a little flirty, but he was always like that, I didn't really notice. After agreeing to go, I deleted his messages for no reason. I delete messages quite often, doesn't matter.
So yeah, we were at his house, watching a moovie, just the two of us. His parents and little brother where out of town, I guess. He didn't really tell me where they were.
The moovie was a boring horror film, even I, who is extra-afraid of these realised it. During the half of the film, he suddenly asked if I would mind if he took of his T-shirt because the heat was too warm for him. I didn't really mind doh. So he was sitting next to me shirtless with some great abs.
About 1/2 hour had passed when he suddenly stopped the movie, put one of his hand on my thigh and leaned closer, asking " Do want to do something else?"
For a moment, I was shocked. The next time, I thought about his girlfriend. I said a quiet "no", and I stared at his laptop. He didn't say anything, but he kept his hand on my thigh for a minute more.
We finished the film in silence, but I could sense he didn't really like my choice.
I told myself I wouldn't talk about this to anyone, and also that it wasn't really a big deal, but I know I would be angry if my boyfriend did that to another girl.
So I'm thinking about telling his girlfriend, but since I don't have the messages he sent me, I don't think she would believe me. Also she doesn't really like me since we were close friends with the guy last year.
And I'm also a chicken and I'm afraid of telling her.
So I'm super confused right now. I'm glad if you could help me, even if you say I should stop caring about them since it's none of my business.
Thank you for your answers ~

I'm probably not the best person to ask about this but I figured I would try my hand at some advice anyway.
The guy didn't try to make a move on you, so even though it's really likely, there isn't actually any evidence from a logical point of view of what he was trying to do.

On the off chance that you tell his girlfriend and you're wrong about his intentions, it might ruin a lot of things for no reason.
On the other hand; if you are right about what he was doing(this is the more likely case), there still isn't much you could do about it. Telling his girlfriend may just upset both of them, and they wouldn't get anywhere, and you may lose a friend. If you think doing the right thing is worth risking that, go for it.

However, either way, I would advice not going to that guys house again.

I didn't talk to the guy after this yet, right now I'm thinking about asking him but there again I'm afraid and shy. We've been quite close before, even if we didn't talk for weeks or months. I don't really want to ruin this, I just know that I'd be upset if my man did this.
+ no one knows what he can to do other girls. I just feel weird being a part of this.
Anyways thank you for your answer, maybe I'm really overthinking it.

As said, you have no proper piece of evidence. So not only can you not prove it, the certainty of the situation is questionable as well. His intentions may be clear, but your credibility towards a girl you aren't on good terms with is unlikely to change something. I would recommend talking to the guy, then again, I don't know how close you are, or if he's a person that would take your words to heart. Nonetheless, confronting him, asking him if he's happy with his relationship, etc. seems to be the best course of action.
Interfering with people's romances when you only know one party is dangerous. I once had a friend who was in a relationship with someone I did not know, but she told me she was unhappy with it. hmmm let's explain the relations shortly: Person A is in a relationship with B, I am a friend of A and C, C is a friend of person B. So I told C that A was unhappy, who then told B how A felt. A and B then had a confrontation, where A denied having any problems, ending with A hating me and B hatin C. This is just an example to show that meddling with relationships you are hardly informed of can end pretty badly.

The best idea is: talk to HIM about it.
say like, hey, what about your girlfriend? I thought you were happy with her
saying something to her will only end badly to everybody

Though this is a fairly useless question, are you a guy or girl? The whole situation depends on this.

lol >>7 honestly >>6 and >>4 pretty much answered in the right way (which is why i didn't comment before) and clearly the OP is a girl.