Guardian and legal status?

I posed a question like this earlier but didn't phrase it right (people get so hung up on little stuff) so let me try again.

A friend of mine who is in collage wants to leave LEGAL custody in her will to a woman who's here on a student visa whom she also wants to make the child's Godmother.

I told her I didn't think that was a good idea (she could still make the woman the child's Godmother, but most people I know like to have both the legal guardian and the Godparent the same person) because if I know the law, it doesn't matter if the woman has an adopted American child or even child born here. She'd not be allowed to stay if her visa expires so if my friend were to die, and the woman lost her visa, the child would have to leave the country with her (since she would now be the legal mother),or cause other legal problems for the child since his new legal parent was not a citizen.

Was what I told her correct? I see to recall people saying a child can't sponsor a parent till the child is an adult, I assume that's true for adoption and legal guardianship as well right?