I would choose to maximise my returns by investing my money where it will earn it's keep.

However, if the Joint tenants are married filing a joint return then technically it is only one taxpayer and not two. 

After a few months there was a disagreement between the two and him and my aunt left the property (it’s been about a month ago now) and refuse to return the keys and remove their stuff.

Just remember that when you do pull money out, you will owe interest on it (~5% more-or-less right now), so whatever investment you put it into, you had better earn you more than 5% return.

Anytime you take money from investors and the returns are made based primarily on your efforts, then you are selling a security and need to comply with securities laws.  

As an example, I personally do not care for oil heat:  I do not like the liability of storing several hundred or thousand gallons of fuel oil (effectively diesel) on the property and, do not trust tenants to keep the tank filled.Regardless of the heating solution you choose, your best return comes from the fuel you do not use (Negawatts or Negabarrels).  

I’d say decent expected returns are 6-10% off of sold price, for a 6 month rehab (more if less), this of course is project dependent.

So there is a good chance they just cut a deal with a vending company and aren't getting a big financial return anyway. 

So many people get hung on the one-time fee and they don't consider the yearly returns from their investment.  

IF you overpay a little on a buy and hold, the return might be a little lower, but over time you will pay down the mortgage, rents will go up and you will have capital appreciation.