Can I change the locks if a tenant moved out but still pays rent?

[Angela Palchowski]

I am a property manager in Pittsburgh, PA and I have a tenant who vacated an apartment in December but didn't tell me they moved until February (our lease states that the only way they can break a lease is if they find a tenant to replace them and pay a $350 lease break fee). They never gave me a forwarding address or anything but they still pay rent every month. My maintenance guy knew the apartment was empty and went ahead and changed the locks to get the new key on the master system but left the door unlocked incase anyone came back. The tenant came by yesterday to show the apartment to prospect tenants and found that her key doesn't work and was irate when she called the emergency line to get in, the maintenance tech told her to turn the handle and that is is unlocked, and now she is also angy that her door isn't locked. She said the prospect tenant is now wary of taking over the lease because of the current tenant having the locks changed without being notified and she wants to be compensated. I don't think this is considered abandonment because she is still paying rent and utilities, but I had no forwarding address or anything. What can I do?  She wants to be able to get out of the lease because she said us changing the locks on her cost the prospect not to sign and is going to cost her a few more months of rent. Side note, in this area we only do leases in the summer time for start and end dates because it is VERY hard to find tenants outside of June-August.