I have a question regarding najasah. If I eat at a restaurant with a kafir friend or family member, am Ii allowed to go back to that restaurant because now I know that a plate has become najis and spoon or dDoes it become tahir?

Yes it is fine to return there. The restaurants wash the plates before using them again.

Also we have a general rule that is to assume everything is pure until otherwise known that it has become impure.