Said the main cause of the issue was a clogged up air filter (I know, my bad for not reminding my tenant about the importance of regularly changing filters (it's written in the lease that he must replace them every 3 months and that I'm not responsible for bills resulting in replacing filters)). 

We also replaced the roof for $7,500.

After getting buckling in just the sunny area, I had to remove the product and replace with something higher quality. 

Cash flow is definitely important (and I don’t know your finances) if you need to replace something major like a roof and you’re never making that money back.

In other words, I'm not going to remodel the kitchen but I'll replace a broken garbage disposer. 

You need replacement windows if it's what I think it is.

Can't they use replacement value if there is a building? 

Strong cash flow, appreciation, and delta between purchase and replacement cost.

The “cost basis” is a little complicated, but essentially you’ll owe taxes on....1) the depreciation recapture and cap gain you deferred on the sold property, plus2) depreciation recapture and cap on the replacement property you are now selling.Remember, a 1031 simply Defers reporting of the gain.

I would ask the girlfriend to replace his half the deposit.