Affiliate Marketing Secrets

I am interested in obtaining search results showing a list of independent fitness retailers and gyms in the US. Is there a search term I can use to keep these results as niche and narrow as possible?

I am a web designer and have a client who owns a DBA of a business (martial arts school) called (for privacy we’ll call it) XYZ Martial Arts. He owns two gyms in NY and had a partner for one of his stores set up his hosting and domain name for his website years ago. He’s recently had a falling out with this person and is no longer is affiliated with the other gym which has a totally different name now. Unfortunately when his partner set up his domain name, he registered it under his name. So he is now using XYZmartialartsny.com and OFFERING ALL THE SAME services to the TEE. Since the site is old i’ts been up for a long time and ranks OK. But if someone gets referred to my client and does a search by name… the other school comes up as if it’s my clients and the user would have NO idea that it’s not the school they originally searched for. This guy can say that he was affiliated with them and direct new clients to him. This I do NOT believe is legal, as it is completely misleading users. Does anyone know anything about this? Is there anything I can do legally, quickly and easily to get my clients name back without a hassle? I’ve been going nuts and no one seems to have a clear answer.
Thanking anyone in advance who takes a minute to guide me.