I don’t think Costco will help with fitting a different person other than the original patient.
An example of how tuff they are is I bought my wife’s KS9 aids. A couple months later I happened to be going by Costco and took my wife’s aids with me to get them cleaned while in that area. Costco didn’t want to do this for me, only my wife, the patient. When I told them I bought the aids for her they changed their mind and serviced the aids.
This was at a San Antonio Texas Costco.