how to file 864 when your wife is unemployed and you need to joint sponsor?

[mier]

Hello there,

A friend of mine is married to a us citizen but his wife is unemployed and his father-in-law will be the sponsor.
his father in law is married and owns a business, filing taxes together with his wife.

can his father in law be a sponsor without having his wife to sign on the 864? or because he is married his wife has to sign?

what documents from the irs does the father in law need to proof his income in case that he owns a business?

thanks!