About two months into the lease they got into a car accident and the boyfriend was injured.

This group has been so encouraging thus far, thank you allWe're terrified (strong word, but perhaps appropriate for us) of what might happen if a tenant were to injure themselves at our property.

What would you do if after someone signs a lease they become injured and receive a service "animal"?

It can hurt communities, and people don't like it when investors injure that communal benefit for their own benefit.

Then the husband was injured and couldn't work for a week. 

If that logic is true, then I guess that means that if the tenant has any other crazy thing in the home, I'm liable if one of their buddies injures themselves on that too.I'm not saying that isn't possible, but I am questioning how our legal system allows that kind of silliness to go on.A landlord should not be liable for any toy some tenant's friend hurts themselves on.

My husband is injured and we have service dogs.

Such as his condition and whether he was badly injured or whether he is working.  

If the tenants, their children or pets were to be injured by items that the owner left behind for the tenants to use, the owner can be exposing themselves to litigation and lawsuits.

You don't want the tenants to injure themselves or their lawnmower by getting too close to them, so take some preventive measures.