Exactly. If the 50/50 split is on paper, and signed by both parties, then Kyle is basically fucked. It literally doesn't matter of Jeremy "wrote any code" or whatever during his short time working with Kyle.
This was supposedly in place before the vesting schedule. Vesting is designed to limit, not give, shares to owners. So if he never signed anything and was part of the company when it formed and it was agreed that it was a 50/50 split I don't believe that vesting would matter for his position at all.