Friend forgot e-mail for XBL account

[Nostrildumbass]

I have a friend that signed up for a Microsoft e-mail address explicitly to play Sea of Thieves and he failed to ever log the e-mail address somewhere or memorize it. The e-mail account was never used for anything other than signing up and getting into Sea of Thieves. Is there not a single page or app anywhere that allows you to sign in using your gamertag instead of e-mail address? I'm sure he'd be able to get in if he could just use the damn gamertag, and then retrieve/update the e-mail address there.