You would first find out if their current/preferred insurance is an allowed substitute (through an employer?). There are circumstances where that's fine. And it matters if they want to delay both parts A and B?

Google this as a place to start:

can I delay medicare

As long as they follow through when circumstances change, they can "play by the rules" and avoid a life-long penalty.

The requirement to sign up is different than IRMAA. IRMAA is based on a 2-year lookback, and it can be appealed. Example: You sell a house with a large taxable gain or win a nice lottery/scratcher, but otherwise live mostly on Social Security, you can appeal the IRMAA increase, if it is clearly a one-off event and a bit unusual and won't affect your typical income.