Having rental property, I will throw my opinion in here.
It is hard to find "good" tenants. Not sorta hard, but actually really hard. I have never even thought about restricting someone's rights in that regard. But I am a member here so that is somewhat of a no brainer.
We use a "boiler plate" lease agreement for our tenants. It would not surprise me if this is a boiler plate. I would recommend your friend ask about this clause and see if they would waive it. If he is squared away and has a good background, I wouldn't think it an issue. Like I mentioned it is hard to find good tenants and we will bend over backwards if we find one!
Tell your friend that oftentimes, there is no insurance coverage for tenants belongings. We always recommend to our tenants a "renter's policy" for their personal belongings. For a gun(s) in the above situation, you might have to get a speciality company to write the policy?
Hope this helps.