The 0.4% deal applies to Billy, as well. As I understand it, everyone is entitled to 0.4% of whatever venture follows from the work they'd collectively do over that weekend. It wasn't Billy who offered it, it was a friend of OP, one of the devs. Any of the devs could start a company as well and all of the other team members including Billy would have a 0.4% share in that, too. In fact the most sensible thing to do is for the 4 members to say 'let's start this thing up, take nearly 25% each, give Billy 0.4% and do our best'. They'd likely beat Billy in competition as they can iterate on the codebase, Billy can not without hiring a new team. If the devs aren't interested in running that startup then they'd still be entitled to 0.4% of what Billy is going to run.
(edit ignore the exact percentages, they ended up with a team of 8 devs than 4)
(edit ignore the exact percentages, they ended up with a team of 8 devs than 4)