Well at least they have music in common. Isn't she from a musical family, plus that is her profession.
Music seems to be his only interest and she can share in that. I know Linda Thompson was supposedly involved in song writing (which as I said before, she never did since - which makes me think he just gave her some credit) but McPhee works with him and performs with him. So that could sustain them.
Plus they have had a over decade long friendship. Remember they were such good friends he even performed at her first wedding in 2008!