I ended up capping the Piano skill before I finished Milestone III. With the Mentoring component you'll have quick success just taking control of another Sim in the household and having them Practice with an instrument. Switch back to your musician, click your target and choose Mentor. Going to the nightclub and waiting for another Sim to sit down at the piano - well, all I can say is good luck with that. I actually moved in my Sim's Romantic Interest just to finish up this portion of the final Milestone. Get your target Sim's needs high if they are under your control so you can have a nice long Mentoring session.