Do you think it is a bad thing that he likes to play alone and doesn't get it on with her very often?
Masturbation versus intercourse are two very different things, and for most people are done for different reasons. It's a bit of an antiquated notion that someone will always choose sex over autoeroticism. People in very satisfying sexual relationships will also masturbate. The two serve different psychological needs.
In this case, though, that's not the entire answer.
Think he is cheating on her?
I have no idea.
But is does seem clear that since they do have a sexual relationship, and that it was much more active in the past, that he doesn't have a lot of interest in her sexually. Unless there is some additional influence going on, like guilt from tyring to be a Good Witness.
I don't know the ages of the people involved, but there are a variety of possible reasons:
Lack of sexual interest (psychological or physiological condition)
Emerging sexual identity issues
Evolution of the relationship
Guilt (religious or non-religious)
Another relationship
Stress
Fear of emotional entanglement/commitment
Self esteem issues
Substance or alcohol use
Other medication use
Could be a lot of things.