I agree with the kiss on lips as friends bit. Did his girlfriend find out? Annoyingly, the most moral thing you could do is to force him to tell her but i know few of us would do that, and maybe not even me (talk about hypocrit!). I have to confess to doing something similar but the girl i kissed was about to finish with her boyfriend. I think that makes it slightly better but i would still have rather not done it. I think any form of 'adultery', whether you are the aldulteror or the guilty accomplis, is best avoided.
On an interesting, philsophical, moral side note, adulterous sex with protection in marriage is not logically wrong. There is no logical reason the partner should mind. Man, i hate philosophy, it makes me feel so archaic and insignificant.