I think you were correct to suggest an alto.
“She also would like to be able to play informal jam sessions with some friends who play guitar and such. For that reason alone, I told her that she might prefer soprano, as I feel any tunes/songs she wants to play along with may fit better on an instrument with a bottom note of C?”
I don’t agree with you here. I mean, it’s not hard to play in the key of C on an alto – in fact it’s almost as easy as F. And other easy and common keys on guitar like G are very easy on the alto too. If she wanted to jam with me, I’d run screaming from the room if she showed up with a soprano! 😉 So if her hands can handle it, for sure go with the alto, IMHO.