How to get my friend to see he's in an unhealthy relationship?

I know you really care a lot about your best friend Jon, but the best advice I can give you is to stay out of it. Why? Because it's obvious Jon is blinded by this woman's affections. So whatever you say, will be lost in space and will go in through one ear and out the other. Also, if you try to come between the two, they might be able to fight, but surely after they reconcile, then you'll be viewed as "the bad villain of the drama" and will be labeled as a nosybody while they are hugging and kissing and everything is peaches and cream. Thus, you can end up being considered as an enemy by your best friend Jon and lose him for good and I know you don't want that. So, I'd stay out of it if I were you. Allow Jon to learn through his own mistakes through trial and error.