Short answer. Either one. Under Article 2072, if you became a guarantor at another person's request for the debt of an absent third person, then after paying the debt you may sue for reimbursement either the person who requested the guaranty or the debtor himself. The choice is yours.
What the law says
If one, at the request of another, becomes a guarantor for the debt of a third person who is not present, the guarantor who satisfies the debt may sue either the person so requesting or the debtor for reimbursement.
Civil Code, Article 2072 — Guaranty at Another's Request. Read the full provision →
The situation the article addresses
The article covers a specific triangle of people. If one, at the request of another, becomes a guarantor for the debt of a third person who is not present, the guarantor who satisfies the debt may sue either the person so requesting or the debtor for reimbursement. There are three actors: the debtor whose obligation is guaranteed, the person who asked you to guarantee it, and you, the guarantor. The distinctive feature is that the debtor is not present — you did not deal with him directly; you stepped in because someone else asked you to. Having paid the debt, the question is who owes you back, and the article gives a generous answer.
Two people you can recover from
You are given a choice of defendants. Ordinarily a guarantor who pays looks to the debtor for reimbursement, since it was the debtor's obligation he discharged. Here the law adds a second target: the person who requested the guaranty. You may sue either the person so requesting or the debtor for what you paid. This is a real alternative, not a sequence you must follow: you are not required to exhaust the debtor first before turning to the requester. You pick the person from whom recovery is more practical — which, where the debtor is absent and possibly hard to reach, is often the requester.
Why the requester is also on the hook
The requester's liability makes sense given his role. By asking you to guarantee an absent person's debt, he set the arrangement in motion and induced you to take on the risk. It would be unfair to let him prompt your exposure and then walk away, leaving you to chase a debtor you never dealt with and who was not even present. Treating his request as itself a source of the reimbursement obligation reflects that he, as much as the debtor, is a reason you paid. It also gives you a solvent, identifiable person to pursue where the debtor is out of reach, which is precisely the vulnerability the article guards against.
Proving the request
Because everything turns on the request, the practical key is being able to prove it — who asked you to guarantee the debt, in what terms, and that the debtor was not present in the dealing. A written request, messages, or the circumstances showing you acted at that person's instance are what let you claim against the requester rather than only the debtor. Keep, too, the proof that you actually paid and how much, since reimbursement is measured by what you laid out. With both in hand, Article 2072 leaves the choice of whom to pursue in your hands.