Give it a try, let him dress as a female, invite him (as her) to shopping, dinner, bar, dancing. Who knows, maybe she'll turn out to be a very special gf with a bit extra. If she wants to start hormones, don't stop her - hormones' effects are slow and you both will have plenty of time to get used to the new changes, or stop taking them... And she's a new person for you, maybe you'll fall in love with her again, if not - you aren't obligated to stay, you can remain friends. But I'd suggest to give it an honest attempt.
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