Who bears the loss of goods that went missing after the partner arranged for a carrier for them, guaranteed their safety, and later refused to bear his share of the loss?

If your friend helped you but did not guarantee the carrier, then nothing is incumbent upon him, due to the absence of the three reasons for guarantee: contract, possession, or damage. The reasons for guarantee are: direct damage, causing damage, or untrustworthy possession. Consequently, the guarantee of the goods rests solely upon you, and your friend’s bearing of a quarter is out of help and donation, not obligation.