guarantor advice?

It is you personally who are on the hook for a loan that you have guaranteed. Credit checks are on people not on cards. Indeed you can cancel that card, get a new number and you have no duty to notify the lender.

If the borrower defaults and the lender tries to come after you via a charge on that card, he will find that it is invalid at which time he will contact you and ask you for the money. You are legally obliged to pay and he can get a county court judgement against you if you do not.

It is almost always a really, really bad idea to guarantee a loan. Anyone who needs a guarantor is not financially strong enough to take on the debt and you can assume the he will therefore default.

You are a fool. It is a scam. Nobody signs up to be a guarantor and being a guarantor has nothing to do with cards of any kind.

If you sign up then that is it you are on the hook for their debts