Female relationships at that age start to become complicated. At least the other girl has been somewhat honest about what might be going on here. It sounds primarily like jealousy, which the other girl needs to learn to handle in a more appropriate manner. I have seen this sort of thing manifest as simple nasty behavior without any apparent cause (even with a possible attempt to get others involved in a perceived fake "slight"), rather than more direct confrontation about the true source of the upset - which may simply be that the other girl is feeling insecure or badly about herself. I am guessing that from your daughter's reaction that this individual may even have been considered somewhat of a "friend" at one time? What I would hate to see would be your daughter feeling the need to hide her gifts just to appease this child. Easier said than done, though. While reasonable modesty is a smart way to go, it should not extend to performing less well than she is able to. Best of luck - your daughter may be in for shades of this behavior for years to come.
