A Squirrel Ripped a Screen - Tenant's Responsible?

[Dan L.]

My tenant put a window AC in. The window screen above the AC unit is now ripped. She is claiming that a squirrel climbed onto the AC and scratched the screen and ripped it. I have no idea if they did it while they put the AC in, or if there really is an evil ninja squirrel. Sounds like a good excuse though. How do you draw the line as to what to bill the tenants for (ie, use their security deposit for) versus just writing it off as bad luck. Their lease is not up for another couple months if that has an impact.