as for walking a serval being legal, the federal requirement on exotics is SUPPOSED to be that they be kept for an educational purpose to include exhibition. most people fail to meet that requirement. but if this person is trying to follow through on that requirement, this may be one way they prepare (socialize) the cat for public education exhibitions.
Where is there a federal requirement on the keeping of exotic pets? There are huge exotic animal auctions in states like Missouri and Texas where anyone can buy a lion, tiger, bear, serval, etc for a few hundred bucks. You don't have to be licensed to buy one. Some states have regulations, others have none. I'm not aware of any federal regulations on who can keep a wild cat as a pet. Places that exhibit have to meet USDA standards (which are stupidly low), but not private owners.
The two servals I've known have been jerks, but they were both in a sanctuary, so they had sketchy pasts. Having interned at an exotic refuge, I have issues with ownership of exotic cats, but putting that aside, I'd agree with the majority of the people here. If this was a public park and not an off leash dog park, then she has just as much right to walk her cat there as I have to walk my Akita. If you have your dog off leash anywhere other than a designated off-leash area, then you'd better have some other form of real control, which means they shouldn't be approaching leashed - or otherwise controlled - dogs (or cats).