Half and half. If an F2P account is linked, and the player logs into the P2P and has the heirlooms, then those heirlooms will be available to the F2P characters.
They will not, however, retain any of the enchants, as each character literally makes their own copy of the heirloom when they click on it in the heirloom tab. It even says "Made by xxx" on the item.