I have been asked this question many times before. The general answer is that he owes all of the taxes because he signed the return. For a joint return the signers are jointly and severally liable for the tax. This means the IRS can collect the entire tax from either or both, but the whole amount is only collected once. If one spouse earned all the money for those returns, the other might be eligible for Injured Spouse relief. If the IRS agrees, he would be absolved from paying the tax.