Tenant has brought in a roommate without permission

[Carlos Lez]

I rent to students, by the room and on their own individual leases. So a 4 bedroom house has 4 people and 4 leases - they are all private rooms and single occupancy. 

It was just reported to me yesterday that one of the tenants has brought in another person and he is living with him. This is direct breach of the lease. I am seeking help on how to handle this situation. He is out of work and has been paying each month from his unemployment money albeit late. He is already playing a racism card. Please see the recent discussion here for context.

https://www.biggerpockets.com/...

With eviction moratoriums going and given the fact that his lease ends in August, I want to be careful in handling this in a way that it doesn't become costly. Any actionable suggestions would be much appreciated. The property is in San Jose, ca.