Well, first of all
It seems that you look like a good friend to involve yourself to see how you can "Fix" the relationship

Anyways, id suggest that maybe Mutt should "ask" Jeff if he would do some sort of Relationship counseling; if Jeff refuses or delay's or just puts it off

then maybe Mutt should try to look for another someone in his life

I try to not look at the negative sides of relationships, But is there a "chance" that Jeff could be seeing someone while Mutt is sleeping or not at home??

if Mutt is your friend, Keep supporting him during these difficult times and tell him that your looking out for him; reguardless of how it turns out