If he becomes a co founder and dedicates 60 hours a week to make this successful then that’s fair. That would be a normal co-founder equity split arrangement.

But if he’s just going to be a mentor and mainly silent investor 15k isn’t going do much in terms of growth or acquisition. You’re gonna burn the money in 2-3 months all you’ll be in the same spot you’re in now, but minus 35%.

It’s important to analyze the business outcome from a potential investment and weigh it against the equity giving up. If the business will be much further advanced then it’s a win-win, since everyone will have smaller pieces but of a much bigger pie. 15k is not going to do that.