Locking Egress Door

[Justin Swann]

I just bought my first duplex and we'll be living in one apt (closest to driveway) and renting out the other apt (furthest from driveway - one has to walk past our apt to get to theirs). Our issue is our dog. We are going to fence in our front yard and lock the gates so no one can open the gate and thus lose the dog. We were also going to also lock their front door to avoid the dog going into their apt.  He's a lab and is quite curious and fast. Their apt has a back door which they'll use all the time. It also has windows on either side of the locked front door which they can egress out if in case of an emergency. My question is if there is any law/fire rule against locking the front door if they have another door and other paths of egress. It's essentially like their front door doesn't exist. I'm being over cautious here and believe there to be no law but want to be cert.

Thank you,

Justin