Generally, a pre-nuptial agreement will not “protect” a future spouse from bad debts, as the premise is not correct.
Debt incurred in one person’s name is that person’s debt alone. So regardless of the terms contained in a premarital agreement, the debts of one spouse do not become the debts of the married couple — those debts stay as the responsibility of only the person who incurred them.
Further, distinct and separate credit events will not affect a spouse or future spouse; debts that are held singly (instead of jointly) affect only the person incurring such debt.
Debt incurred by BOTH the husband and wife AFTER marriage affects both spouses, however.