If the old telephoto lens fits her Rebel, it will for your 5D.
But if it's a really old FD lens it won't fit her rebel or your 5D.
There aren't lenses specially made for the 5D, but there are more expensive lenses usually used with full frame cameras like your 5D.
I would consider getting lenses if your mom is going to get upset at you stealing them

But it depends how much money you have for lenses.
And what you want to photograph.
Can you say what lenses you're going to be borrowing from your mom?
And what you want to use them for?
That'll give a bit more idea for getting you the right information...