There are two basic problems: format and regional code. Format (NTSC/PAL, etc.) was mentioned above. Besides this, there are "regions" (http://hometheaterinfo.com/dvd3.htm), which are supposed to keep people from buying new movies on DVD before the movie actually opens in local theaters. America is Region 1, Europe is Region 2.
What you need to do is ask your friend if her player will play NTSC and ANY REGION. I have heard from people in England that new DVD players will play NTSC disks. Also, I am an American living in Japan (Region 2) and most of us buy inexpensive "regionless" DVD players, which will play all regions. So smart people are finding ways to get around the stupidity of not being able to play DVDs from home at your overseas home.
Anyway, ask your friend what her player will play. If you plan to send her lots of DVDs, get her to buy a "regionless" player that'll also play NTSC, if possible. If not possible, then you'll have lots of converting to do.