Short answer. If he acted in bad faith, he forfeits his own goods in the mixture and owes you damages on top. Article 473 does not give him a share proportioned to what he contributed; it strips him of what he put in, and leaves you a claim for the loss the mixing caused you.
What the law says
If the one who caused the mixture or confusion acted in bad faith, he shall lose the thing belonging to him thus mixed or confused, besides being obliged to pay indemnity for the damages caused to the owner of the other thing with which his own was mixed.
Civil Code, Article 473 — Mixture by One Owner. Read the full provision →
Two very different outcomes turn on one fact
The article splits at good faith. A mixture caused by one owner honestly — stock tipped into the wrong bin, grain delivered to the wrong silo — is unwound by proportion: each owner ends up with a share matching the value of what he contributed, and nobody is punished. Bad faith removes that arithmetic altogether. The party who deliberately confused the goods keeps none of his own, and the loss is his alone. Everything therefore rests on proving intent rather than on valuing the two lots, which is the opposite of what most people prepare for.
What forfeiture actually means for the mass
He loses the thing belonging to him thus mixed or confused — the goods he introduced, not merely the profit on them. The practical consequence is that the whole commingled mass answers to you, including the portion that was physically his, because his interest in it is gone. That is a real remedy where his stock was inferior and the blend is now worth less per unit than your original: you are not reduced to owning a proportional slice of a devalued mixture, which is exactly the result a deliberate mixer is usually hoping for.
The indemnity is separate from the forfeiture
The article says besides, so damages are additional and not absorbed by what he forfeits. They cover the harm the mixing caused rather than the value of the goods themselves — the difference between what your stock was worth unmixed and what the blend fetches, the cost of any testing, sorting or reprocessing attempted, and losses flowing from a sale you could no longer make or had to make at a discount. Each of those has to be shown; forfeiture is automatic on a finding of bad faith, but the money figure is not.
Prove the intent, and prove what you had
Bad faith is rarely admitted, so it is built from circumstances: access to your storage, the absence of any plausible reason for the two lots to meet, a prior dispute, a warning already given, timing that fits a motive. Alongside that you need a record of your own goods before the event — quantity, grade, assay or inspection results, purchase documents and the price your material was commanding. Take a sample of the mixture early and have it analysed, because the composition of the mass is the fact that ties the two lots together and it degrades with handling.