All you can say is it is a novel idea, but since I have never heard of that combination I have no idea if it would work out.
Is it possible to just run all the Subways tighter and use the combined income to pay off the loan, or sell out of the one location, to at least reduce the loan amount due, and keep the rest??
Tell him he needs to spend time at the Subways to control costs and efficiency. If the owner does not want to be there, or can't, then the managers MUST be on site. Managers typically work 50-55 hours a week on site to control labor costs and teach workers exact formulas for making sandwiches to control costs and to order properly so food costs stay down.
Food costs and labor costs are the things that can kill a business like this. Without active management the workers do what they want, including eating on the job for free and taking food home (theft).
Sounds to me - from your rather sketchy question - that your friend suffers from entrepreneur syndrome, which Steve Jobs famously had. They WANT the risk, they want to live on the edge and have the excitement of starting a company from scratch and all that that entails. And they are poor day-to-day managers, in general. This is a pretty well known set of characteristics. If they ask you for advice, you give it, and they just ignore you then you have done your part. It's their company, they can run it into the ground if they want to and you can't stop them.
I know someone who owns several subways. This individual recently opened a new location with a loan which is not fairing well though is profiting slightly (not enough to pay on the loan if it were the only store). This owner is not managing their money well enough to pay on the loan and thus wants to sell all of their stores and open a new business which is a combination of several services I have never heard of being combined before. To me it seems much riskier than just keeping the subways and managing the money better. This person has been coming to me for advice on the situation and I have been steering them toward just putting more effort into generating more money out of the subways and avoiding risk. Currently the owner and most of the management are hands off doing the bare minimum. I care about the owner and some of the employees and I want the best for them but I don't know what to say.