I think you need to discuss with this person and ask her to put her business plan forwards to you (not her potential earnings part) just the plans for the property part as it may affect your business
and you need to know how health and safety comes into it
you would need her to show you on paper exactly what she plans for this gym room eg , sepearate electrics toilets water?
If you cannot find anything out perhaps ask your local authority at the health and safety department ,
just say alterations need to be made to the premises as gyms may require air conditioning , stronger phases of electric power , and I think maybe a first aider on board at all times ,
you need to know how it may affect your business regards using the water supply or if you need to seperate it
or if she intends to use your toilet for her clients ect,
also the electric needs to be seperated too ,
I think it should be stipulated in a contract that she is liable for all of these things herself ,
pehaps go to a local college where they train up fitness experts
and ask the Tutor can she give you some advice on all of the matters you need to know or maybe she can point you in the right direction
hope this helps
also I think you need legal advice about a getting a proper legal contract getting drawn up ,
hope that made sense