Well that super sucks. You should ask (ask first before you go ninja raiding her room) for BOTH of them back. Friends shouldn't keep all your stuff. My friends and I borrow manga from eachother all the time, but we ALWAYS give it back. We can always borrow it again if we want. Though, sometimes it does take a month or two to get them back.

Then again, I just remembered that I lent my friend a novel (not manga. Just normal book), and she lent it to her friend. Like, 2 years ago!... I wanna read it again! gonk