You really have two forms of payment here given that you can't be paid out of the small sum of money the founder has. The decision as to what to accept depends on the idea the person has and your belief in it.

1. Equity

It's called sweat equity for a reason. If the person you've been approached by is not willing to give you equity to work on the idea it's a major red flag. You should be compensated for your work and if the person is cash poor they still have equity in the company that they can assign to you. This is given that the person who has the idea plans to scale the company and potentially exit it.

2. Delayed cash compensation.

If the person is planning on raising capital you can create and agreement that states that they will pay you once the company has raised capital. If you do decide to accept a delayed cash compensation package, you should first understand if the person is planning on raising capital and how much they are planning to raise. From a founders perspective they'll want to only pay you the delayed cash comp if they raise over a certain amount which is understandable. Make sure you understand what that mark is and that it's attainable.

3. A combination of equity and delayed cash compensation

A combination of equity and cash compensation has less upside then only equity as well as less risk. You'll be paid less because of the equity piece but also have a part of the company for the long term.

With all those options out there it's important to note that the best option for you depends on your current situation and long term interests.