If you are sending the video from the video application via MMS, try a different way. Start a blank MMS message and attach the file that way, instead of sending through the video application. It should send. The size limit for Blackberry devices to be able to receive is 3 mb. Hope that helps.
I don't think there is a problem with her Blackberry. I believe it is a problem with Sprint. I have noticed a lot of other Sprint users have similar problems sending MMS videos and recieving them, according to their forums. You may try saving your video as a 3gp file and see if that makes a difference. That is the default video file extension the Blackberry saves all its video. Keep us updated on your progress.