You'd need his Materia, but if someone had that and the know-how, sure. That's how we cast magic where I'm from - or in this case, summon up guys like Bahamut. Summon Materia are pretty rare, though.

[so she's technically right! Though thinking about it, he wonders where his Materia ended up after . . . hm, back to the Planet, probably]

I beat him both times. [he grins at her] But it's never permanent, so I'm definitely not the first or the last to do it.