Have they just been friends or do you feel their was more going on? He shouldn't be lying about speaking to this person. That is not being honest in your relationship. Is this person someone he talks to when he is having problems like a friend that he goes to for guidance, advice?
My partner is 30 years older than Seb (the friend) and they are both pianists so they met in a musical setting. He was introduced to me as someone who could work with me professionally (I am a singer). They fell out for about 6 months after which Seb asked to resume the friendship. I don't feel there is anything sexual (that would almost be easier in a way) but it is the emotional attachment and the sycophantic nature of the relationship that disturbs me. It left me feeling like an outsider and rather paranoid at one stage. I also feel that Seb is a manipulative character. Accepting money and gifts from someone else's partner and keeping yourself in the picture even though you are part of the problem is inexplicable. I just don't know if I should leave. He goes to Seb to feel dominant and have his ego stroked.