I work at a PATH Premier Center, and am getting my own PATH instructor certification. It's a voluntary organization, and by becoming a member, you agree to maintain a certain set of safety and horse care standards. To gain certification for your program, you agree to an on-site "inspection" by a team representing PATH. To gain instructor certification, you take a series of written and in-person tests, along with teaching and riding demonstrations. It's a rigorous process for centers or instructors.
That said, it's completely voluntary. Anyone can say they're offering therapeutic riding, but it's up to participants to do their homework and understand if the people offering the program have appropriate credentials. I'm willing to believe that there are qualified people running therapeutic programs who don't want to deal with the time & expense of gaining PATH certification, or maybe just don't know about it. But as a consumer, I would want to send my child or family member to a certified center because of the standards that I know they would be meeting. That's why PATH Intl. and member centers do work hard to educate the public on what attaining certification really means.
One other thought, while I am not currently living in a place where most insurance companies will pay for therapeutic riding or hippotherapy, I did formerly volunteer at a PATH-certified center that served many children whose insurance covered their lessons; I believe in that instance the insurance company required certified professionals leading the hippotherapy (i.e., PTs & OTs) and that the center itself needed to be certified.
I definitely don't think this place is doing anything "illegal," but I personally would approach what they're offering with a degree of skepticism and would want to know a lot more about their qualifications and future plans.