If she ever wants to improve it's better to be honest, but you can be honest and not be a piece of shit about it as well. It might hurt her to hear if she's got any kind of misplaced confidence but if it pushes her onto the correct path to legitimately get better then it's a net gain. Just playing along could lead to complacency and a backwards slide in performance when she thinks she's already good and tries to push harder but just ends up sounding worse. Depending on how serious a matter it is, if she's lead to believe she's good and then someone else whose opinion she might take more seriously tells her she's bad, it could become resentment for lying to her. Also depending on how menhera she is, could go far enough to twist it into thinking it was intentional sabotage or, my personal favourite, that you were secretly making fun of her the whole time and didn't really support her because you're actually just jealous she's making progress in a career path you had offhand said once was something you'd thought would be fun but didn't have the talent for.

It's really fun try it sometime 10/10