I would keep track of the revenue and expenses by asking him how his business is going periodically over a beer and not investing in it at all.
Even though I'm comfortable with my original answer, I'll elaborate because this comment gets to the heart of the matter.
you noticed details I didn't even think about, it's my first time, I get the feeling that I'm going to be screwed due to my lack of experience eventually, but I'd like to give it a try
There's a huge, massive, incomparably different financial commitment to starting a business versus buying stock in a public company that there should be two different words; because they're not both "investing" (even though obviously they're both investing).
I'd venture that all of your investing activity to-date involves buying some publicly traded and regulated securities. A publicly traded company with reasonable volume means your money can go in and out whenever you feel like it, practically speaking. Feel like owning Apple for an hour, no problem. When you buy shares in Apple, you're buying them from some other person like you (or entity or pension fund or whatever)
When you "invest" in a start-up your money gets spent. You hand $10,000 to your friend, it buys a POS system or pays a contractor to paint or install something or pays an employee, or whatever; it's gone. Your money isn't sitting in an account waiting for you to retrieve it. It is NOT like buying Apple. You get maybe some paper that says you own X% of this thing that spends money; neat. What happens when the company needs more money to pay another contractor, or permits, or a bill? Is there an operating agreement that attaches an obligation to you for capital contributions? Will your X% be diluted if, beyond all odds, someone else buys in? Can the business get a loan? Who co-signs the loan? Under what circumstances would the business seek a loan? Who has the authority to apply for a loan? Do you want to allow your business partners the ability to sell their ownership to anyone (hint: no you don't)?
You don't need an accountant, you need someone who has opened a restaurant from scratch before. Your friend managed someone else's restaurant that was already running? That's not relevant experience, and you have less experience than that. Paying an accountant to tell you how fast your money was spent will just accelerate the spending of your "investment."
There are a plethora of times where starting a business makes sense, and I'm not even completely opposed to starting businesses with friends, but this is not one of those times. You and a friend have an idea for some project that you can code up and maybe bring to market, but there are some costs related to licensing or hosting or paying a designer or, if things go fantastically well, an accountant to file your taxes, great spend the money that way. Start-ups spend money. It's called a burn-rate for a reason.
And one last point, if you move forward with this terrible idea, you probably wouldn't get screwed. At the end you'll still have have the piece of paper that says you own X% of the thing that spent your money. Getting screwed would be the chef embezzling money from you. The failure of a poorly conceived start-up does not constitute screwed. It's really unlikely for this idea to get to a place where there is money to be embezzled.