Who does an E-mail address belong to?

My friend’s ever-so-mature manager was mad at him so she fabricated a complaint E-mail about him.
It was shown to him, and he printed a copy.
The main giveaway of it being fake is that the e-mail that it’s from does not match the from name or the address written at the bottom of the E-mail beside the name. (It ends by saying, with the name changed, "From Sue Hamburger, sue.hamburger@hotmail.com)

Now I want to prove that this E-mail address belongs to her!
My best idea so far was to start a fake e-mail account and pretend to be some person she met somewhere. If she responded I would have proof! But she may not be quite that dumb.

I’m wondering if anyone has a better (but still legal) idea that will help me find out who owns the address.

There is really no way to determine who an email address belongs to but you can find out from what I.P address it was sent from if you have all the header and that should be sufficient.

But you have to have the entire original email because it is trivial to fake it.

If you have the original IP address it was sent from you can go to whois and find out who owns that domain.