What you had entered into is a conceptual agreement, and is not bound to the object itself.
If you reimburse the money for your half of a new bong, and the other party refuses to purchase another one with you, then the other party is in breach of contract.
In short your contract is to have, and use a bong in a shared environment no matter if you have to go through 10 of them.
As a matter of the object itself, if a person is using due care, and using the object in a manner that is consistent with it's intended purpose in the event of accidental breakage no fault shall be incurred.
In the U.S though the degree varies from state to state in order to collect on the broken device some sort of negligence must be demonstrated.
I think you should use any parts remaining, and purchase whatever parts together that are required to make it work again, even if it is a lesser system (remember it is not the object in question here it never was), and honer your agreement as friends. If he is not willing then take your loss, and re-evaluate the friendship as required by your own personal belief.
Personally I think that the guys instant dissolution of your agreement, and demand payment for the object seems like a red flag sign that your dealing with a douchebag.