The attitude should *almost always* be to refuse.

App ideas are easy to come up with. There is 0 value in them, and they are worth $0.

"Technical co-founder", when you are NOT the one who started the business, is another word for "sucker". If you are a technical person, came up with an idea, and reach out to someone to help with sales or marketing or operations or whatever... that makes sense. If someone reaches out to you and says "I have an idea..." and they don't have cash in hand to pay you plus equity... wish them good luck and move on.

J.Ja