Two foreign language booksellers we buy from at our library:
http://www.grantandcutler.com/ - mainly European, but they stock a range of books in other languages too.
http://www.mantraling...portal/bilingual.html - predominantly children's books, but some of the older children's titles may be suitable (depending on how old your friend is).
Both places, if they can't help, may be able to point to you to other sellers. It's also worth trying the local library - they may have sources you never dreamed of.