you already seem to be sensitive to being polite about it, and, frankly, nice enough to want to give her crit rather than bitch behind her back, so i'd say you're already on the right track. all it will take is being polite and upfront.

although, to improve how it's taken, be sure to also point out what you DO like about her playing. try to find examples of ways she's done things that you really thought were good, or IC, or gave you a lot to work with as a tag, and frame it in a "i'd love to see more of this" way.