How much would you pay a fitness trainer in this business situation?

If it were my business, and if I wanted to give the new hire a cut of the action they produce to keep them motivated and producing, I’d ask them what they expected. If they want almost the entire pie, they are telling you they aren’t going to be happy with any reasonable offer, and will eventually depart with a bevy of students so they can have the whole pie to themselves. I’d think that given the costs of marketing and providing space, accounting, risk management, etc.; 50/50 is extremely generous. Your trainer friend might surprise you and ask for considerably less that that.

Wishing you the best in growing your business.

I would also think about paying based on how many people show up. Someone could pay the monthly fee but if the trainer is bad they won’t come back next week or re up next month. If you pay X dollars per head you motivate them to keep the program interesting and treat your customers well.
I know that’s how people are paid in the yoga studio I go to. The more popular teachers get paid more.

Firstly you should not mix business with pleasure. Do you want to keep this friend? If you really have your heart set on doing business with him make sure you write down whatever terms you two decide. I would give your friend and hourly rate unless they are going to do recruiting too.

What is standard practice in the industry? 35% does not seem like much for having supplied the customers, set up the sessions, etc.

If you’re doing the marketing and taking care of all of the overhead (and thus, risk), 50/50 is way too generous if the class is large, and very risky for your friend if the class is small. Personally, I would not offer a set percentage, but instead a guaranteed fair per-class compensation with small incentives for certain class size milestones. This lets your friend avoid the risk of an empty class and thus feeling hosed if he only gets a cut of 2 or 3 people, but lets you really profit if the class is large. Also, by offering incentives for larger classes, your friend is motivated to help bring people in. Everyone wins :) Just make sure that your reasonable expected minimum number of students will cover what you’re offering your friend, in addition to your overhead for the time the class is running.