In the classes I teach, I find that this is a frequent problem with women. A lot of women, when they extend their arms all the way, will hyperextend. In other words, rather than their arms being fully extended in a straight line, their arms bend back the wrong way a little bit. This throws the inside bottom of the elbow right into the path of the string. In addition to what Arne said, if you find that your girlfriend has this problem (it's easy to see if she does), then make sure that the doesn't lock out her arm in the fully hyperextended position. Have her stop extending her arm before she gets to the locked out position. Unfortunately, a normal arm guard won't help, because the part of the arm that gets hit is above the part that any normal armguard protects.