Eh, Star Wars has always played it pretty heavy with "the will of the Force." Luke one-in-a-million shot at the Death Star towards the end of ANH, for example. It's not like Rey had an easy life growing up as an orphan on a poor desert planet as a scavenger.
It was clearly demonstrated in the beginning of The Force Awakens that Rey is proficient with a quarterstaff so I had no trouble accepting she could pick up a lightsaber and grok it immediately. Quarterstaff to lightsaber should translate more easily than quarterstaff to longsword—eg no worries about edge alignment. Especially considering force sensitivity and all that pizazz.
I'm not actually against this decision at all, I kind of liked the idea that he wasn't ready to live up to his potential. A hero in a trilogy needs their journey from farm boy to knight, and I was really ready for focus on the villain actually growing in ability. It would be hard not to like an antagonist with well written drive that we needed to see on screen, and we nearly got this too.
I didn't particularly like the force awakens in cinema but that's mostly because it didn't do anything new, except have a villain who both started as a villain and had a lot of potential to grow without redemption, which would have actually been kinda new for starwars where basically every character arc with growth is a good character corrupted or a bad one redeemed.
They explained that in context. He told Rey she needed a teacher, and she used the Force Knowledge Extract trick he unwittingly taught her when she was a prisoner.
Watch the fight again... She's getting her ass kicked, doesn't even know how to hold a lightsaber.
"You need a teacher..."
"You're right." Closes her eyes, force downloads the techniques Matrix style, THEN kicks his ass.
Also Ren had just killed his own dad and survived being shot by a gun that launches everyone else it hits clean off their feet. He wasn't exactly on his A game for that fight and the film made that clear
Even wounded he could have force choked her walked over and stabbed her through the guts with his lightsaber.
This would be a much better ending, because we all know getting stabbed by a lightsaber in Disney Star Wars is barely a flesh wound and she could be back for the next movie.
I still think Finn should have turned on the lightsaber, given it a dramatic twirl, and immediately cut off his own leg just to demonstrate how dangerous lightsabers are.
I mean... he picked up the lightsaber, dramatically went one on one with Kylo Ren and immediately got the shit kicked out of him. I thought that was fairly sensible.