For me personally, it would depend on the circumstance, person, and product. You are going to be an investor quite literally in the product so you need faith i

For me personally, it would depend on the circumstance, person, and product. You are going to be an investor quite literally in the product so you need faith in the founder(s), the mission, the business plan (if there isn't one, you need to tell them they need one), and you need the deal structured like a cash investor would have. If they balk at any of these asks or push back, you should walk in my opinion. I've done this before but on a very informal basis. Would I ever do it again? Probably not. It's just too easy to get burnt or all your work to go to waste (which is what happened to the one time I did this). Are you choosing not to be a co-founder or are they asking you not to be? Why? These aren't questions to answer here, but simply asking yourself.