whatalicedidnext
I, personally, would suggest you to read the trans widows thread.
The fact that this boy is attracted to women, makes it more likely that he has AGP, rather than gender dysphoria.
This is difficult for you, because you don't want to alienate your daughter.
And you don't want to hurt her, either. Both entirely understandable.
But you sound like you have a good relationship with her. It would be useful for you to be able to spot signs of AGP, if this chap has it.
And, if she starts to express doubts, you could get her to read the thread too.
Either way, for all the reasons mentioned here, this is probably going to be a bumpy ride.
Distressing to contemplate, I know.
But I don't see the alternative.
If it goes well, then it's a plus.
www.mumsnet.com/Talk/womens_rights/3101834-trans-widows-escape-committee
I'm also linking to a thread which explains about the two types of men who transition to being women. As background. Perhaps read that one first.
I'm sorry to sound gloomy.
You sound like a great parent. 
www.mumsnet.com/Talk/womens_rights/3145470-Break-it-down-for-me