Maybe waste $50 buying an ill-fitting crap bike in order to learn what's wrong and right. Then know, adjust, and buy again.
I have to disagree with this idea. I think there are a few possible pitfalls:...so I agree, buy a cheapo used bike on CL and take it for a tune up at your local bike shop.
1. For someone who doesn't know anything about bikes it is very likely they are going to buy a used piece of junk that has a lot of problems.
2. Where I'm at, the $50 - $100 used bike plus basic shop tune up will cost as much as a new bottom of the line hybrid from Target, which is a better bike than anything you are likely to get for $50 - $100 used, even after a tune up. If the bike needs any major parts it can get worse.
3. Putting a new cyclist through the process of navigating buying a used bike, getting it repaired at a shop without ending up with a fortune in it, and then having the cyclist find out that riding the bike sucks because it is still a 1970s 10 speed Schwinn tank that doesn't fit them and has an uncomfortable seat, aggressive geometry, etc. sounds like a way to turn someone off from cycling forever. It will work for the most determined but many will give up.
For the purposes of the OP I'd recommend finding someone to borrow a bike from. Friends, coworkers, neighbors probably have a bike that is gathering dust that could be borrowed just to relearn how to ride.