A couple of things I've picked up over the years.
- The property owner or the one who holds the title is the owner de facto or the owner in fact.
- Your friend is the owner de jure or the owner under the law.
- A lease is a contract or an agreement between two parties where the de facto owner, in exchange for payment, grants the de jure owner certain things. All provisions must be agreeable to both parties and be legal.
Now, and here's where your friend needs to read the contract's fine print, and, maybe, talk to an attorney. If there was nothing in the contract prohibiting firearms your friend may be able to sue, or threaten to, for breach of contract.
A contract can't be changed in the middle of the term just because one party feels like it. The time to change a contract is before both parties agree to it.