Client wants me to be a cofounder

I work for a small software shop. One of my clients offered me a position as a cofounder after we'd worked together a bit, but I turned him down (all above the board and we just continued). I've even tried setting him up with another developer who he can trust, but he wasn't interested.

Yesterday he let me know he was open to basically any arrangement (equity and/or cash). He's my favorite client, we work well together, and I love the project. Still, in my heart of hearts I know I don't wanna leave where I'm at in the immediate future/go anywhere else full time. So we arrived at something along the lines of I work exclusively on that project one day a week, or whatever.

He doesn't want to hire anyone else at the shop I work for, and it would be easier for him to just pay me directly. That would be sweet, but its weird. Fundamentally though, direct to me or through the consulting company, I want to see the project successful. The referral came directly to me from a good friend. My company definitely doesn't want to give up any of my time, but at the end of the day I'm responsible for me so it's my call.

Not really sure what to do, on super good terms with everyone involved & my policy is & has been to keep everything out in the open.

So...take a 1/5 paycut and go on the side? Go through the company and let them chomp away a percentage without being able to contribute? Do it on a 'saturday'? Honestly I can probably start managing projects & instead of doing them and hold everything down (at the expense of some personal projects).

I'm new at this, I like building stuff and I don't have very much wisdom. Any advice or stories is appreciated!