I have a question about when you would culturally use different levels of politeness in Japanese.
Would you use formal or informal speech when speaking with a really close friend's parents?
My best friend moved here from Japan a few years ago and her parents can only really speak enough English to function, so if I talk to them I have to use Japanese. However, I'm always worried I'm being too casual/too formal depending on how I'm speaking.
I know there aren't really set rules for that type of situation, but is desu/masu form too formal, or is infinitive/casual form too impolite?

Thank you!!