Need to know how to handle a tenant moving in/out mid-lease

[Hugh Jameson]

I have two tenants for a condo.

each paid half deposit/1st month etc..

1 tenant personal situation has changed and he is considering leaving mid-lease.

the remaining tenant has a partner who is willing to move in as a replacement so to speak. 

does this constitute a break of the original lease and a new 12 month with the partner re-instated?

does the tenant moving out pay a pro-rated amount of the lease remaining to break it etc...? 

some guidance here would be super helpful 

TIA