These rules regarding the commuting expense have grown complex over the years with many exceptions. 

What level of outside income does he have to support it?

Well below 1% rule.

Hello, Quick question,  what is the 50%  operating expense rule  when it comes to small multifamily properties . 

All are in C or B neighborhoods and they all are between the 2% and 3% rule.

This evaluation is further supported by a recent CBRE pricing opinion of $11.5-12M"What they done is evaluate the price by assuming the return is 5% annual.

It does not meet the 50% rule, but the calculator shows that it still has a positive cash flow and ROI.

I was hoping there was a good rule out there, amount of equity amount of REI, amount of "x" before you make a move.

To give solid advice the community will likely need more information, (where you want the vacation rental, how much you think you want to pay, any questions/ concerns you have) It is difficult to give advice with no supporting information. 

Are there rules around taking out a home equity loan on a rental property that would prevent this?