Postage ~ I should probably know this already

[kuhio98]

So, I want to send a package to a friend in Canada. She is short of funds, so I want to enclose a SASE (self-addressed stamped envelope) in the package so that she can reply without having to pay for postage.

So, my question is.... Do I need to put Canadian stamps on the envelope? If I put U.S. stamps on it, will she be able to mail it back to me from Canada?

Don't know how I made it 47 years (and living in 6 different countries) without knowing this.