There's nothing "opinionated," nor funny, nor "just an exaggeration," about what this coworker is doing. She's not stating an opinion, she's not trying to be funny, nor have some "exaggerated" fun.
She's passing off a false statement as fact, (something which CAN be definitely proven or disproven by testimony of her physician), and doing it maliciously. There's nothing in the OP's statements that implies her coworker isn't seeking to be taken seriously. The OP sounds very concerned about her reputation, and IF this coworker IS just exaggerating at the OP's expense, she's picked an inappropriate subject matter with which to engage in workplace antics. (HIPPA protected information in the workplace), so a nice cease and desist letter would end that bully's bullshit.
There's a REASON health information is a legally protected class of information. The LAW KNOWS how damaging health information can be, and it's forbidden to disseminate it or use it as a "joking tool" in the workplace, so really, this coworker needs to be educated by the supervisors on numerous levels.
Now, what a judge considers opinion vs slander vs "just a joke your honor. We didn't mean anything by it" is a pretty easy solid defined line, and the "it's just an opinion. It was just a joke, a prank" defense is only used by low-grade defense lawyers who are going to lose, and who disrespect the intelligence of the judge they're standing in front of by using such a lame defense in the courtroom and trying to pass it off as law.
Also, the coworker is also not going around talking to coworkers constuctively, ie saying something like, "I THINK this girl may have an anxiety disorder, try to help her feel at ease," (but even THAT can be legally risky), so the "I was just looking out for her wellbeing and she misunderstood the whole thing. (The gaslight defense.)
She's being malicious and saying it was THE REASON she stayed away from her, intending for others to follow suit. I hope you're not a civil rights attorney. You do realize young people kill themselves because of workplace harrassment stuff like this right?