Mark, Chris' take on this song is that it is about Adultury and there is always another man, or woman involved, I don't think it is about any one person in particular.
I think its always easy to make assumptions that songs are about people close to them, and often they are, especially his family ones about his brother , his children and his parents, but quite often they are totally unrelated and an idea just springs into their mind.
Its a great album and one well worth getting.
Laureen