When filing a joint return, your son and his spouse essentially become one taxpayer. So if one files an extension, it'll apply to the other when a joint return is later filed.
His wife will not need to make a payment and she isn't required to file a separate extension.
Making a payment is one way to file an extension, but if his wife wants to cover all bases, she could file her own extension without a payment for free HERE or by mailing in a Form 4868 with 0s for all of the numbers.
If they reside in a state with an income tax, be sure to check state requirements for an extension as well.
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