This community is meant as a place where travellers can be helped by other travellers. We are not NS employees, so do not have access to customer data, and cannot help you with your problem.

You should contact customer service directly. This can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook.

Contact NS indeed, but from what I understand the following is the case:

When you link somebody else’s card to your Mijn NS account, the owner will be informed by e-mail that somebody else (you) linked their card. Provided that the card was already linked to another NS account, of course.

This could be useful e.g. when an employer needs insight into your travels, or make a declaration for (some of) your travels.

The e-mail contains a button to ‘delink/undo’ that anytime (i.e. when not wanted/needed anymore), after which the card should disappear from the other person’s account upon next login.

Apparently you (or your friend) did that but their information still shows up in your account? In that case, try logging out and back in.