While there are many ways to skin this cat, it seems like the right way to approach it is to consider the offer from your POV, not from an obscure universal concept of fairness.
Assuming you believe in the idea and in the ability of the two people to execute, what do you think the valuation will be at the time of acquisition? I know it's black magic, but suppose you arrive at a number of ten million. Once you arrive at this number, make an estimate for the time frame of the project. Suppose you decide the acquisition will happen in two years. Is 2% of ten million ($200,000) after two years of work for no pay acceptable to you given the high risk of the enterprise? If so, and you feel you won't be able to make more money elsewhere, take the opportunity. If not, look for something else.
The valuation and time frame estimation is voodoo, but figuring that out is a whole separate issue.
Assuming you believe in the idea and in the ability of the two people to execute, what do you think the valuation will be at the time of acquisition? I know it's black magic, but suppose you arrive at a number of ten million. Once you arrive at this number, make an estimate for the time frame of the project. Suppose you decide the acquisition will happen in two years. Is 2% of ten million ($200,000) after two years of work for no pay acceptable to you given the high risk of the enterprise? If so, and you feel you won't be able to make more money elsewhere, take the opportunity. If not, look for something else.
The valuation and time frame estimation is voodoo, but figuring that out is a whole separate issue.