I would think that the moment you're offered 40% of a company, your first thought would be "this guy clearly doesn't believe in this company himself, or else he'd never offer me that much of his company". There are so many options available, be it leaving no stone unturned to find some additional cash (bank loans, borrowing from family, cashing investments, etc), or offering a non-equity profit sharing arrangement, that there's really no excuse for giving up large chunks of equity until you're receiving serious capital in return. This just sounds like a stupid deal to offer, and a worse one to take.
Being the Chief Tech at Conceivian (and having the best job of my life), I can tell you that working with startup founder's is a thoroughly enjoyable experience, and I'd do it again. There were tons of learnings, innovation and fun in the project Andrew mentioned and several others we have done. BUT nothing beats getting fully paid for the work done!!