To tell or not to tell

I met this guy Jason at a party two years ago. We kissed before I went home that night. Anyway, I only got to see him once after that before he rejoined the army and I had to go back to college. We kept in touch and actually I’m not talking to him at the moment. Why? Well Almost a year after we met he told me he was interested in other girls. (One of whom is a good friend of mine Jenn Hoy and this other girl named Claire.) I told him I couldn’t compete with friends for that. Jenn Hoy finds another boyfriend so she’s happy now. Jason and I decide to be friends. Anyway, he tells me that he got stationed in Ft. Lewis. I thought that was cool because I go to college in Washington state. Anyway I see him when he gets there and we end up making out. But he still says friends. Anyway we keep in touch after that. Then I find out some friends of ours got stationed in Ft. Lewis too. ( Aaron and his wife Amber.) So the four of us hang out. He makes out with me again and still says just friends. Time goes on. I get added to Jason’s Yahoo 360 page. I’m bored and I’m looking at who Jason has on his buddy list and I come across Claire. I click to go to her page and I read some of her blogs. Jason is the only guy she writes about. It says they’re in a relationship and she went to see him this past July. After they spend the week together Jason says we’re friends with benefits and that he and Claire are in an open relationship. ( Meaning they can see other people.) I ask Claire if it’s the truth because of what he said to me. Claire doesn’t email me back. Instead I get a phone call from Jason saying he’s upset about what I asked. He said it seemed like I was trying to break them up and that luckily he and Claire are very honest with each other and he would never cheat on Claire. However I know he’s been online dating her and he had a girlfriend in Seattle at the same time at one point. This is on top of making out with me mind you. I haven’t really talked to him since. My question is should I completely rat Jason out to Claire so she knows what he’s been doing if she doesn’t know already. Or should I just let her figure this out herself? Im asking because she deserves to know the truth and if he is doing this behind her back she deserves someone better.