Yeah, this crossed over from parents being uncomfortable about their precious boy seeing a married, older woman into parents now overtly controlling and threatening.
Yes, you may want to give your parents a heads up about the situation. It is not that hard to track down someone's phone number on the intertubes.
My only thought is to decline to be intimidated and also decline to engage. They can only out you to your parents and if you have handled that, well, piffle.
But I also don't see a point to talking to them right now. Maybe in the future when they've gotten over the toddler stage of accepting of their child's adulthood. (Seriously, this happens - my mom had a hard time with me going away to college and I didn't come back with a married girl or boyfriend.) They are having a tantrum. Their son is being an adult in a very disturbing way to them and they are likely kinda freaked out that their son is becoming an adult, and not their little boy.
Support karatekid - he'll need it.
I also urge karatekid to stick it out if he can, as un-fun as that is. It's not so easy to find financial support to get through college nowadays. Colleges can help but frequently there is not enough money to go around. Getting through college with less debt is a great gift and a real advantage in life. This does not mean put up with anything they dish out.
People do work hard, scrape by and manage to put themselves through. Many of my friends did. However, I know that the quality of the college experience I had (interactions with classmates, learning opportunies like internships, quality of professors - for example, I was taught by full professors while my friends were taught by adjuncts) was better in many ways than experiences of my friends who paid their own way at public colleges or universities. I admire my friends immensely. But, they had a really hard road. So if karatekid can work it out with his parents - they accept his adulthood and mature decisions gracefully, if not happily - and he continues
his education with their help either directly or with other support, then that would be best for all.