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?

by

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.