That's going to really be a he-said she-said situation. How can you prove that the two companies talked? Perhaps if OP had a job offer and then all of a sudden it was pulled, there could be a reason for a defamation lawsuit.
Remember that saying: never attribute to conspiracy that which you can attribute to incompetence. It's more likely that someone just blabbed about your previous coworker and word got back to the boss.
If you choose to interview with some of those other companies, you're going to have to do it SUPER stealthy. No correspondence or phone calls during work hours. Don't even sneak off to take personal cell calls. Interview after-hours or on weekends if possible.
Explain the situation to the new employer. If they need the help they could really give two craps about the fortunes of your current boss and will help you make the jump. If they do talk to your boss then, well, they probably can't be trusted with other things.