You deserve a special answer because this is a special case:
When calculating equity, you need to put a value to the company.
There are many ways to do this, so the best thing for you to do is do some searching on "company valuation" online.
Generally, equity is calculated based on the post money value of the company.
Meaning, after you invest, how much is the company worth?
For example, say Joe has a company that's valued at $50,000 and wants Bob to invest $50,000 in his company.
The 'Pre Money valuation of the company is $50,000, the post money valuation of the company is $100,000. Therefor Bob's equity share in the company is calculated as 50,000 / 100,0000 = 50%
EQUITY SHARE = Investment Amount / Pre-Money Valuation + Post Money Valuation
It really depends what you bring with your $25,000 and what everyone's expectations, for the company and for each other are.
You haven't really given much to go on, so it is hard to give a meaningful answer.
Yeah this is a depends question for sure.
You are putting in the $$$ he is putting in the effort. You are taking more of the 'risk' in my opinion and at some point there may be an option to sell or bring on other investors to get your initial investment back out. Or maybe not.
If you want to be able to say, it's time to sell, or we should bring on another investor, and are going to have an active role in the company then 51% ;)
But if you are just sitting back and your friend is extremely competent then 30%
Y Combinator popular software / web app VC firm gets around 8% for their ~15k investment in startups. But they have tons of apps to weed through and they invest in only the best options. They also hook them up with great contacts and aren't very greedy about it.
I think you should be looking ~ 20% - 30% for a passive role at this stage. Unless you want to control your money and investment, then 51%
When my startup was funded with $150K, the investors received only 10% of the company. It's less about the amount of money and more about whether or not you are the first to invest in your friend. If you are: 10% is a good starting point. If you aren't, you'll likely get less. Not every investor can receive 10% of the company.
Basically, if your friend is pre-seed, you are his round A and 10% is fair. You can get more, and being your friend, he might give you more just to make sure the shares are in the hands of a friend to prevent hostile actions later on. You can ask for more to cover your risk if you believe your friend will fail or mishandle the money. But then, why would you give him the money in the first place if you didn't believe in him?