The landlord had no reason to enter unless the landlord knew the tenant was in jail or has not paid rent or the landlord believed the rental was abandon. Either way, if the landlord knew the tenant was in jail, then the landlord should have served the tenant in jail. If the rent was not paid, then the landlord should have posted notice and proceed with eviction process. If the landlord believed the tenant had abandoned the rental the the landlord should have changed the locks to prevent anyone from entering the rental. The landlord did not change the locks and the tenants friend re entered the rental. The tenant might claim construtive eviction be clause the landlord failed to reclaim the rental by not changing locks and the tenants friend entering showed that the tenant had not abandoned the rental. It always pays to follow the correct proceedures and the law to the letter. This landlord has left him/herself open to legal action.
The law doesn't require the LL to change the locks, although common sense does. It comes down to the timeline involved. If the tenant has been absent from the property long enough, the LL may have taken all the appropriate legal steps to reclaim the property. We'll just have to agree to disagree. I prefer to believe that the LL did the right thing, rather than imagine that the friend of a jailbird knows the whole situation and has related it clearly here.