People grossly underestimate how understanding people will be if you just explain your thought process in a calm and reasonable way. If she asks why he went along with it if he thought that’s what she was actually doing:
“Look I’m not about the burning books thing but I thought you were cool and I figured I’d just go with the flow and see how things went, worst that could happen is I don’t hang out with you again 🤷♂️. I’m glad it’s a regular party though, let’s go hang”