It's hard to see how they can guarantee that you won't be diluted.

There's a lot of variability in these kinds of negotiations. If you really think the company is going places, negotiate for as much as you can and say "yes" before it puts a strain on your relationship.

FWIW, if the other cofounders are close to having customers and a functional service, I think their offer is in the ballpark and you could easily justify taking it.