Is it permissible for an employee to work diligently during the agreed-upon working hours (8 hours), then work without interest during the overtime hours that he is forced into without right or compensation?

What your friend is doing is an act of injustice and oppression. He must fulfill what you both agreed upon regarding working hours, as Allah, the Most High, says: "O you who have believed, fulfill [all] contracts." It is your right not to work the extra hours.