Hi,
I made two playthroughs for that fight:
1st, i joined the prince before facing the queen. After the fight, i have access to his store.
2d, i defeated the spider queen before talking to him (to try to have more xp as the game warned me that "accepting help lowers XP by 40%" in the tutorial village iirc) and talked to him after the fight... same discussion as in 1st playthrough, but then he disappears and his store is not accessible at all because the trigger fight is already done.
Isn't this a bug?