Dude just call me and let me know
Seriously though, he has to know if his daughter really is the weakest player on the team. Just let him know at the end of the season that the team needs to be more competitive the next season and that maybe his daughter would be a better fit on another team where she would get more playing time. She needs to develop more as a player and on your team she would see a lot of bench time next season.
BUT....I know your a coach of a younger team so Im not really sure why it's that big of a deal to let the player stay. She's still young and unless she can't catch a ball or she swings the bat like a flyswatter you don't know if she can turn into a quality player or not yet. Try working with her and explain to the Dad/Coach that with a lot of work and time she could turn into an every game player but next season she might see less time on the dirt.
Who knows, if he's that much of a nice guy maybe he thinks you suck as a coach and is just waiting for an easy way out.
My daughter is perfect, she knows everything. Just ask her.