You will meet new friends no matter where you go. Staying in a place that's bad for your horse is unfair to the horse you are currently responsible for. If he needs more turnout then you need to find a way to get him the turnout he needs and meet his other needs.
Rudeness is inexcusable and unprofessional. You don't say what she did to treat your horse badly-"poor treatment" obviously can be wildly subjective.
You don't describe what you are supposed to be getting with the boarding fees. Is it just the stall? How much turnout? What are you paying for- and what aren't you getting?
You cannot worry about someone else's horse. There will be plenty of creatures in your life that you get attached to- you cannot control what happens to most of them because they aren't yours. You already have one horse that you are having issues in terms of meeting his needs- unless you can safely say you can meet the needs of both animals you can't buy the mare (even if the owners would sell her to you,) then you need to take her out of the equation.
Part of riding is developing a relationship with the horse- that will happen with ANY horse you ride on a regular basis; it sucks, but you need to prioritize what's best for YOUR horse rather than thinking it through with emotion.
Do what you feel in your heart to be right- for you'll be criticized anyway. You'll be ****ed if you do, and ****ed if you don't. - Eleanor Roosevelt