Let her play FH if she enjoys it. One day (in the not so distant future) she won’t have any more sports to play and she will be working for a living like the rest of us. Are there any other softball players on the team? If so, could they play catch right after practice every so often just to keep the feel?
I normally would agree with the sentiment in the first sentence, but in this case, a teenager who (as the OP states) has clear aspirations to pitch in college might need some adult guidance. This might be a scenario where just "do whatever is fun" isn't the best advice for someone who is driven to achieve a goal. She might have to sacrifice field hockey to achieve that goal, and while ultimately playing or not should be her decision, I don't know that it's bad for a parent to lay out the different scenarios here.
IOW, saying "If you truly want to pitch in college, you may have to give up field hockey this year to dedicate the time to achieving your goal" is not such a bad thing.