It's not just that it might be viewed as a hobby, but "equine" is notorious for being one of the "businesses" which caused all the "hobby or business" stuff to be created.

Horses are a cost center and suck money like a sponge. Your client might intend to make money, but the more they delve into trying to make that a business, the higher the expenses will go. You are going to need to learn about the livestock regulations, the care and feed for my own horses vs boarded horses,  the "of course that show trip for my daughter to ride hunter-jumper is part of business." Etc.

I would start by interviewing the customer, to see where they intend to take it: breeding, boarding, training, retail feed and tack, transport (I have a friend that makes lots of $$$ hauling horses to events for owners), and will help be hired, or boarders who work there get credits, or school girls who want free riding lessons (risk management issues). That will tell you if you have a future with this client, or it's just rainbow sparkles and unicorns in the sky.