If he's offering you such a high salary, that shows he's very keen to have you. And you mentioned he's wanted you for a while. Which might mean you have lots of

If he's offering you such a high salary, that shows he's very keen to have you. And you mentioned he's wanted you for a while. Which might mean you have lots of leverage, even if your friend wouldn't want to admit it. So even if he would prefer you to focus on his thing, you could just state that working on your product on the side are your conditions for joining his company. And see what he says. If he says no (and you feel he really means it, and you feel you really do want to work on his project full-time) almost certainly the 400k full-time offer will still be on the table.