Many good people have had their credit ruined through divorce or medical bills.

The rare few ruin it for all but make sure in the future you do your due diligence!  

It is something they took non chalantly and stated, maybe your get paid this week but maybe not.Realize they are doing technical changes which will in long run really ruin them. 

Not only that, but they sometimes use the wrong products and end up ruining surfaces.

I haven’t had any carpet ruined by peeing so far. 

What is the toilet floods and ruins the carpet?"

and if the house is relatively new or thoroughly remodeled, that could be lower as well, but a tenant can ruin even a new house rather quickly.

I would hate to have to do this as it would likely ruin our friendship, I'm having a hard time finding other options.

At the same time, the debt of the refi and the new debt of the 4 plex will be a big hit to my debt-to-income ratio - it may ruin my ability to use conventional financing.Outside of the property I have in contract there is not another property I would like to purchase right now. 

Yeah, it might suck taking it head-on but if you keep letting this drag-out next there it will be 2 tenants doing such, and so the "cancer" will spread.