I actually disagree in this case. Normally, buying cheap and learning are good things ... but this guys is going to spend $200 on a bike and be over it in two weeks? Then he has to go out and get the $1000 bike he should have bought originally ... and he has the unused bike sitting around. Or ... he spends $1000 on a bike and if he only rides it three times, sells it for $800 in two weeks. he tosses in the accessories for half-price (assuming the shoes and helmet fit the new buyer) or Ebays them.
But ... I see no reason to think this guy is full of it. I see no reason to think he is suddenly going to develop a passion for MTB and hate road-riding. (Not sure I would recommend MTB for someone with a bad knee anyway.)
On top of that a $200 used bike will likely need work and parts anyway---tubes for sure, likely tires, probably cables ... which means he might need help adjusting the brakes and shifters. So ... a trip to the bike shop.
On top of that he needs to find a cheap but not garbage used bike, in his size, in his area, in good shape ... I watch the ads where I live and you know, I don't see many options, and I can do all the repair work myself. I look for any bike which isn't crap and is cheap, because I could swap out the entire group set off my spares shelf if i wanted. I still don't see many bikes worth the effort.
The last thing this guy needs to do is get a crappy bike and have it not work well, seize a bearing, not shift properly ... and what if the wheels are not round and true? Not properly tensioned? Not odd on a $200 bike. All that means time and money at the bike shop, and less enthusiasm about riding. And if this guy knows next to nothing about bikes ... how could he tell what he was getting?
I'd rather he get a $125 Walmart bike if he was only planning to ride it for a season anyway. For $75 more at the LBS he'd at least be pretty sure it would last until the snows returned. A used bike is a crapshoot.
So ... let him go to the bike shops. Let him see the price tags. If he is scared off, oh well. But maybe, Like Any Of Us, he will see the bikes and get excited about riding one regularly. Maybe, Like Any Of Us, he would picture the place he could ride, he would imagine himself going farther and faster ... maybe he would imagine one day buying one of those really hot-looking but pricey bikes and doing centuries or multi-day rides.
Just Like Any of Us.