Should work. My wife uses one on her Cowon. For the same reason as your sister would.
My sister purchase a RCA Opal 2gig player. She loves the display, It is big and bright. She can only she out of one eye so that is important to her,, but it needs to be charged by way of the usb port on a computer. She doesn't own one!! She had a old mp3 generic player that charged by way of a usb connection to a AC power adapter. I was wondering if that would work. I called RCA support line. They really don't know much there. I really not sure why it wouldn't work I would think the power would be right. As long as the connection would fit.
I gave her a old laptop that was only able to run win98. The opal needs xp. But with laptop on it was able to charge the player. She is planning on doing alot of traveling here soon and doesn't need to lug around a heavy old laptop just to charge the player.