Short answer. Both you and the lender share the cost equally. Article 1949 of the Civil Code provides that when extraordinary expenses arise from the actual use of a borrowed thing, even if the borrower acted without fault, those costs are borne equally by both parties — unless your agreement says otherwise.
What the law says
If the extraordinary expenses arise on the occasion of the actual use of the thing by the bailee, even though he acted without fault, they shall be borne equally by both the bailor and the bailee, unless there is a stipulation to the contrary.
Civil Code, Article 1949 — Extraordinary Expenses in Commodatum. Read the full provision →
The rule for use-related extraordinary expenses
Article 1949 of the Civil Code distinguishes between two types of extraordinary expenses in a commodatum — a loan of a thing for use without compensation. Expenses for the preservation of the thing are generally refunded by the lender, provided the borrower gave prior notice. But expenses that arise from the actual use of the thing are different: if the extraordinary expenses arise on the occasion of the actual use of the thing by the bailee, even though he acted without fault, they shall be borne equally by both the bailor and the bailee, unless there is a stipulation to the contrary. This 50-50 split applies even when you did nothing wrong.
Why the law splits the cost
The logic behind equal sharing is that the lender benefits from the loan arrangement too — they have not lost ownership and the borrower is using the thing on their behalf in a sense. When something unexpected and expensive happens in the course of that permitted use, the law does not place the entire burden on the borrower who was acting properly, nor does it place it entirely on the lender who gave up possession. The 50-50 rule reflects the shared nature of the commodatum relationship and prevents either party from being left with an unfair loss from an event neither caused.
What counts as extraordinary
Ordinary expenses — maintenance, regular upkeep, the costs a borrower incurs simply by using a thing — fall on the borrower. Extraordinary expenses are those beyond the normal range of what it costs to use and maintain the borrowed item: sudden necessary repairs from an unforeseen event, emergency interventions to prevent loss or serious damage, costs caused by circumstances outside ordinary use. The character of the expense — genuinely exceptional rather than routine — is what triggers the equal-sharing rule.
The agreement may say something different
Article 1949 makes the 50-50 default subject to any stipulation to the contrary in the parties' agreement. If the commodatum was formalized in writing, it may already address who bears extraordinary expenses and in what proportion. Check the terms you agreed to before assuming the statute's default applies. In the absence of any such agreement, the equal division the Civil Code establishes is the governing rule — but the parties were free to arrange it differently, and many lenders and borrowers do.