Don't delete the user yet.

Can you check your other tabs and make sure you don't have any invalid fields that are empty? Are you getting an error message in your admin logs?

Dear Stephen,

Yes, the email in the shipping tab and billing tab gives an error warning.
I have changed the email to an valid email address.
After this I was able to change the order to incomplete.
Now everything is ok.

Is it safe to delete this customer now?

Regards, Ton

Yes, you can just perform a normal "soft" delete of the user. Don't do a hard delete because that would not work since the order is still tied to the record in the database. There is no need to perform a hard delete.

Thanks stephen,
Have a merry Christmas for all of you.
Regards, Ton

Merry Christmas to you and your family too!