How about the fact that he loves her easily as much as Tom Cody does?
Yeah, on the one hand, it's kind of weird since Fish is a bit sexist at times towards other women. But on the other hand, you'd be surprised how many female fans Rick Moranis has.
Ellen probably just wanted someone who wasn't Tom and could talk business.
Lots of long hours working together. Things happen.
Not to mention we're only seeing Billy Fish during a time of crisis. When things aren't as tense, it may well be that he's an honestly caring and supportive nice guy.
On the other hand, it's said that the reason Ellen and Tom couldn't work was because he wasn't willing to come 2nd place to her music, tagging along behind her. Billy, her manager, would be just as focused as Ellen on putting her music first.
What's Tom Cody's problem, anyway? Why can't he have a relationship with Ellen?
He knew he and Ellen "were going in different directions", and wasn't the kinda guy to be a groupie. Ton figured the best he could do for her was to let her go and stay with her music.