Not a bad list, but I feel it’s missing something. And since I’m a helper, allow me to assist:
7. Say “What the fuck is wrong with you?”
If you’re at the store with said friend, say it loud. If friend is showing you the costume at home, say it loud. If they show up wearing it, say it loud every time you run into them while they’re wearing it.
Bonus help: “Accidentally” spill your drink on the costume. Now they have to change. How helpful of you! You’ve prevented personal embarrassment of being the friend of a complete toolbox.