Allowing a guest to move in with Tenant?

[Chris Allen]

Background: Just closed on my first househack duplex where I inherited tenants. The tenants are a married military couple who had just extended another year prior to me closing the property. I have already talked with them and they are okay with signing a lease with me as long as they can still keep their current terms, i.e pets, rent, lease term, etc...

Problem: My tenants have a friend who is going through a divorce and wanted him and his dog to move in for a few months while he gets things figured out; I told them I am okay with this as long as he passes a background check. Now they are wanting him to stay there for the full year, as my tenants husband just got orders to ship out for a year and she will still be living in the property. She does not really want him added to the lease. 

Question: How should I handle this? Assuming the friend passes my screening; should I draft a new lease for my tenants and just add him to it? Should I draft a new lease for my tenants and just give a written agreement for a long term guest? Just add some sort of addendum for him or what? Also, since he also has a dog, which will make 3 total, tenant has 1 service dog and one "pet" dog, should I make the guest give a security deposit and a pet deposit/fee? I want to protect both myself and my tenants, so that if my tenants decide they dont want him there anymore, he has very limited "rights" to the property, while also making sure I am protected.