You should have received an email for this ticket. If you forward the email to him, he can open it on his phone to load the eticket in the app. Etickets aren't linked to your Mijn NS account, so simply logging in with your username and password won’t work.

Did you not receive an email? Then contact customer service to have them resend it. 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 X (formerly Twitter) or Facebook. Phone is probably fastest.