--On Thu> Try the following (and replace with the correct URL):
$ ldifmodify -x -H ldap://localhost/ -D cn=config -W << EOF
dn: olcDatabase={0}config,cn=config changetype: modify add: olcRootPW olcRootPW: {SSHA}cZbRoOhRew8MBiWGSEOiFX0XqbAQwXUr EOF
There doesn't appear to be an old olcRootPW value either, so that wouldn't work.
Generally, they'll need to export their DB via slapcat to an LDIF file, and then either add an olcRootPW value to it and re-import it, or add a SASL mapping for the root user so that EXTERNAL works, and re-import it.
--Quanah