Short answer. Probably yes. A depositor must reimburse the keeper for loss arising from the character of the thing left with him. You escape that only if you did not know and were not expected to know it was dangerous, if you told him, or if he already knew without being told.
What the law says
The depositor shall reimburse the depositary for any loss arising from the character of the thing deposited, unless at the time of the constitution of the deposit the former was not aware of, or was not expected to know the dangerous character of the thing, or unless he notified the depositary of the same, or the latter was aware of it without advice from the depositor.
Civil Code, Article 1993 — Depositor's Liability for Dangerous Things. Read the full provision →
What the rule is actually doing
Nobody in this situation set out to hurt anyone. A friend agrees to keep a box, a drum, a container of chemicals, a battery pack, a firearm. Something goes wrong and he is the one out of pocket. The Code does not ask who behaved worse; it asks who was in a position to know. The person who owns a thing knows what it is, and the keeper — who agreed to hold it, often as a favour — usually does not. So the loss arising from the character of the thing goes back to the owner. Read it as an allocation of an accident, not as a finding that you did something wrong.
The three escapes, and how narrow they are
The article lets you off in three situations. The first is genuine ignorance, and it is doubled: you must neither have been aware of the danger nor have been expected to know it. That second limb is the one that catches owners. A person storing his own fuel, solvents, batteries or ammunition is expected to know what they do. The second escape is notification — you told him. The third is his own knowledge, however he came by it, without advice from you. A keeper who is himself in the business of handling such goods will often fall into that last category.
What you owe, and what you do not
The obligation is to reimburse loss arising from the character of the thing: the fire it started, the corrosion it caused, the harm it did to the place where it sat, the cost of dealing with it safely. It is not a general guarantee of everything unfortunate that happens on the keeper's premises. If the damage came from his own carelessness in handling an item he had been properly warned about, or from something wholly unconnected with what you left, the article does not reach it. The link between the thing's dangerous character and the loss is the thing to examine first, and usually the thing actually in dispute.
What to do about it now
Warn him immediately, in writing, even if it is late — a warning given today can prevent tomorrow's loss, and it does not worsen your position on anything that has already happened. Say what the item is, what it can do, and how it should be handled or where it should not be kept. Offer to retrieve it or to move it somewhere suitable. Where loss has already occurred, get the facts recorded early: photographs, the incident report, the repair estimates. Losses of this kind grow the longer nobody names the hazard out loud, and it is easier to settle a documented one than an argued one.