Thanks for reaching out Ian,
I understand the frustration. If your friend uses a @yahoo.com address for his Microsoft account, he should first visit account.microsoft.com to verify he can log in. If successful, clear the browser cache or try logging in via the Xbox app or Minecraft launcher.
If he cannot log in, he should reset his password using the Forgot password link. Two-step verification may also be a problem.
Does he receive any error messages, or does the login page simply load continuously?
Regards,
Rez