@Joe P., I had a similar incident a few years ago with a SFR. Thankfully with a SFR. No bug problem before. A few months in, wife complains of bugs. I check it out and the kitchen was not quite filthy but generally unkempt. Trash overflowing and dishes with left-over food all over the place. They had three children, including two very young children, two jobs and a PT job, and I think life overwhelmed them.
When I talked to them about the need to keep the place cleaner and the roaches being a result of the trash and food, they seemed sincerely oblivious to the possibility. It was as if they had never put the two (dirty kitchen + roaches) together in their minds. Their thought process was "see roaches, call landlord for pest control appointment." I told them to clean and told them to spray for bugs themselves.
They never really got the place clean and the bug problem never disappeared. Not long after, we mutually agreed they could move out early. I called a pest control company and the guy said "No problem." Two visits in three days, $75, and all I had to do was sweep up a lot of dead bugs. Years later there have never been roaches again.
The problem you face is it is a MFU. In your case, I would give the tenant a week or two to clean adequately and have a pest control company (at landlord's expense) treat the unit and bottom unit. If you treat only the top unit, the roaches will migrate to the untreated unit. That's the last thing you want. That's why I would not make it the tenant's responsibility to pay -- it he doesn't want to or can't afford to pay, it won't get done. If the dirty tenant does not comply with cleaning, see if you can evict him -- I don't know PA law, but hopefully there is something in the lease he has violated by being so filthy. Or, if the lease is close to expiring, simply provide notice that you will not renew and tell him you won't penalize him for moving out early.
And I agree with the others who said a pig will probably not stop being a pig. He might be a great guy and an otherwise good tenant, but a pig is a state of mind.