The proof of income for the wife is her personal data. The CU is looking to obtain her data from her husband. Please tell me the legal basis, under GDPR, for the collection of her data by the credit union? She can, of course, consent to the use of her data for this purpose, but there's no basis for collecting proof of spousal income by default for all loans. The Central Bank has even advised credit unions against doing this.You’re just making stuff up!
Data protection, which you mentioned earlier, is of no relevance here.
A lender, when lending to a married person, can ask about household income. A credit union can even ask the other spouse to guarantee the loan.
A guarantee is totally different. The spouse, in that case, enters into a legally binding arrangement promising to take on the debt should the borrower renege on the debt.
A CU can ask what they like, I suppose, and some do still do this but they really shouldn't be asking for income details about a 3rd party who is not party to the credit agreement. It makes no sense. They also won't have sight of the spouse's debt position since they are not allowed to do a CCR check on them (why, you ask? Oh yes, data protection! There's no legal basis to run a credit check on someone who is not party to the loan application).