You have the same choices as you always have even without this predicament. You refuse to play, and whatever consequences happen happen.

In this case, she might end up off the team, a pariah, or whatever but that is superior to a cripple. Heck it might even inspire a few more girls on the team to stand up with her. And we get the same "discussion" started, by them making a huge fuss about it, without having to do it on the suffering of a martyr.

Unlike a lot of avenues, enough forfeits or lack of players will cause an actual effect in the league and demand a response. A place where individual boycotts hold significantly more power than most tranny invasion points.