Originally Posted by mines
These situations are difficult for me. Since she hasn't mentioned the musical, I would not mention it at all. Frankly it doesn't seem that she's very invested in your friendship at all. She appears to be more self-absorbed than anything else (your comment about her being triggered if anyone seems more important....). Ask your other friend to go, and enjoy the musical. It's unlikely you'd run into the 'original' friend but if you do, just smile, say hi, and keep walking.
It seems that original friendship has run its course. And either way, you really don't owe her anything - you made the promise when your friendship was more solid, if I understand clearly. Also it was a "promise" about seeing a musical together - not a promise that you'd keep a deep family secret and now you're telling everyone. There are level of promises. Things change - friends often grow apart. Life is too short to throw ourselves at "friends" when we get little to nothing in return. Just my thought.
Please keep us posted on what you decide and how things go. I wish you the best!