So uh, did this writer just get their own episode's moral wrong? It's like they don't actually know what friends are like so they never considered the obvious solution that makes sense for what's going on.
Consider the facts:
1. This is a school play, with children acting, in a tiny cave set, produced and set up by roughly 8 individuals. Probably written and built in about a weekend. No one expects a fantastic production here.
2. Both the play itself and the invitation for Celestia to star were specifically meant to give the princess something nice for her anniversary. Twilight obviously didn't think that including Celestia would increase the production value of the show, she did it because she knew it would mean a lot to her.
Given those facts, nothing in the episode really makes any sense, especially the conclusion. The Friendship solution, the correct answer the writer got wrong, was to first be honest with Celestia about her acting skills, then proceed to not give a shit and let her do it anyway! The play is celebrating Celestia, and exists only as a kind gesture to Celestia, so no one is going to fucking complain about her acting! It's all for fun, so just let her have fun whether she sucks or not! It really makes it hard for me to view the characters as "friends" when they go "Oh no, she's a terrible actress, the play is ruined" instead of what actual friends do, "Haha wow Celestia that was terrible! Don't worry about it, we'll work on it and everyone will love seeing you onstage."