What they bring is industry knowledge, but not irreplaceable.
They have no financial obligation (to support family) so the refusal to deep dive full-time is more of risk aversion than financial necessity.
Consider this: some (most?) VCs are wary of investing in startups where the founders are not 100% committed (i.e. no other fulltime job). It's worth reflecting on why they would be hesitant about such arrangements, and why you would choose to think differently from your position.
But.
I would set some clear guidelines and deliverables. I wouldn't say "until first sale" but rather "for 4 months" or whatever timeframe you both think appropriate.
I would also discuss vesting and equity percentages. (I would think you'd have a larger equity sharesince you are taking on more risk.) Always a good idea to set up the "divorce" papers before you start working together.
And finally, I'd have good long discussion about why they want to be half in half out of a startup.
On the other hand, I have offered 50% share to people to do little. Sometimes you need an anchor to keep moving, but the part-time co-founder can be a poor anchor.