Chit Chat I.T. help needed re: Outlook email address and display name

This has been driving me nuts for months now so hopefully someone on here will know the answer.

I access outlook online via a browser.

When I start typing a friends name in the “to” box, it shows his correct name in a list which I then select. However, once selected it shows an old display name for him which is incorrect. He also sees the incorrect display name when it reaches him.

I have completely deleted his contact details and added them again as new. However it still shows the correct name in the list as I type his name but once selected shows the old incorrect name!

Any ideas?