You should expect anywhere from 20% - 40% return on your investment into a business (depending on how risky the business model is).
For example, if you invest $100,000 into the gym, you should expect between 20,000 - 40,000 each year in return. (this is separate from what you will continue to get paid as an employee) This may seem high, but it is the standard for what small businesses yield, due to the higher risk. Also don't forget you'll have to pay interest on the money you borrow from your friend, so it's not all in your pocket.
With that number in mind, the next step is to find out if your 100,000 investment in the business would support the kind of ROI you are hoping for.
Let's say for example that the gym has sales volume of 500,000 per year, with a 10% net profit of 50,000 per year.
So the 2 questions on the table in this example would be:
1) For a 100,000 investment, how much of the profits is the current owner willing to give up?
2) Does your share of net profits the owner agrees to equal your goal of 20%-40% return on your investment each year?
Your answer to question number 2 will help you measure if its a good investment for you or not.
Hopefully its a start for you to ask the right questions.