I agree with your first instinct that the purple somehow overwhelms it. Is purple her favorite color? If so definitely use it in a pieced border but I'm not sure piano key is the way to go on this one. I think something a bit less colorful using something from this site would work well, like Rick Rack done with that purple and the white, or tipped bricks using the purple and one of the other colors and white or the Entwined one.
http://www.quilterscache.com/B/Borders.html
If you do go with piano keys make sure you put a few white ones in there too or alternate every other colored one with the white. What is going in the big white squares?