The difference is that good is an adjective while well is an adverb.
Pretty good – adequate quality, suitable, fine, not bad
Pretty well – in a suitable way, nicely, fine, quite
- The party was pretty good last night.
- “Did you enjoy the film?” – “Yes, it was pretty good.”
- “How did the actors play?” – “They played pretty well.”
- Jane can drive pretty well, she’s never had an accident.
As an idiom, pretty well means almost, nearly, almost completely:
- He goes to the pub pretty well every night.
- The new project is pretty well finished.