Is it permissible for a girl to associate with a person who works as a music teacher, knowing that he does not actually teach music during lessons?

Do not accept him as a husband until he abandons his forbidden work and repents to Allah. For whoever fears Allah, He will provide for him from where he does not expect, as Allah Almighty says: "And whoever fears Allah - He will make for him a way out And will provide for him from where he does not expect." So, if he abandons the work, repenting, then accept him. But if he does not abandon it, then not marrying him is more befitting.