Short answer. Only your relative's own portion of the debt. Rule 86, Section 6 of the Rules of Court provides that in a joint obligation of the decedent, the claim against the estate shall be confined to the portion belonging to him — unlike a solidary obligation, where the full claim could be filed as if he were the only debtor.
What the law says
In a joint obligation of the decedent, the claim shall be confined to the portion belonging to him.
Rule 86, Section 6 — Solidary obligation of decedent. Read the full provision →
What the law says
Where the obligation of the decedent is solidary with another debtor, the claim shall be filed against the decedent as if he were the only debtor, without prejudice to the right of the estate to recover contribution from the other debtor.
Rule 86, Section 6 — Solidary obligation of decedent. Read the full provision →
A joint debt: only the decedent's portion
Rule 86, Section 6 states that in a joint obligation of the decedent, the claim shall be confined to the portion belonging to him. Since your relative's loan was joint rather than solidary, the creditor's claim against the estate is limited to whatever share of the total debt belonged specifically to your relative — not the full amount of the loan.
How this differs from a solidary obligation
The same section treats a solidary obligation very differently: where the obligation of the decedent is solidary with another debtor, the claim shall be filed against the decedent as if he were the only debtor, without prejudice to the right of the estate to recover contribution from the other debtor. In a solidary debt, the estate could face a claim for the entire obligation, with only a separate right to seek reimbursement from the co-debtor afterward. Joint and solidary obligations are treated as opposites for this purpose.
Why the distinction between joint and solidary matters here
Whether an obligation is joint or solidary determines how much exposure the estate has to the creditor directly. A joint obligation divides the debt among the debtors from the start, so each debtor — and, after death, each debtor's estate — only ever owes their own share. A solidary obligation instead lets the creditor go after any one debtor for the whole amount, leaving that debtor to sort out reimbursement from the others separately.
What the rule does not resolve
Rule 86, Section 6 confirms the joint-versus-solidary distinction for claims against a decedent's estate, but it does not itself determine what portion of a specific joint debt belonged to your relative, or how that share is calculated if the loan agreement does not spell it out. Those questions depend on the terms of the loan and the general rules governing joint obligations.