Do you think my new bf has cheated on me?

My bf has been away for a week in Ireland with a female friend of his whom he used to go to uni with and has known for about 4 years. (My bf and I only became a couple about 2 weeks ago, and I knew this trip was coming up for him.) He is staying with her in her parents house, which is also occupied by her two brothers who he is also good friends with. Before he left, he told me not to worry as its a platonic friendship and that they sleep in separate bedrooms. I talked to him online and over the webcam whilst he has been staying there, and when i asked if he has ever been more than friends with her he told me that they were never together, and that they DID kiss once on a drunken night out, but decided it was better to stay friends from that point onwards. He told me not to worry cause he would never cheat on me and cause they dont want to be more than friends, and cause whenever they go out, they have guys with them too. He only started going to visit her about a year and a half ago. I know that he was engaged to his last gf and that she cheated on him twice, and he has said he would never put his partner through that. Based on these facts, is it likely that he hasnt cheated with her whilst he has been away? (there are pics of them on his facebook where he has his arm around her and they look pretty cosy but im sure she is like his best friend). He has both of us on his facebook.