How Can I Recover Payment for Shares That Were Delisted and Converted to Cash in Nigeria?

Your assessment is largely correct. There are really two separate issues here:
1. Dangote Flour Mills shares that were converted to cash
Dangote Flour Mills was acquired by Olam International and eventually delisted. Shareholders were entitled to receive the cash consideration approved for the acquisition.
If your follower never received the payment, he should:
Confirm the exact number of shares he held at the qualification date.
Contact the company’s registrar that handled the acquisition payout.
Provide:
CSCS statement (if available)
Share certificate or evidence of ownership
Valid means of identification
Bank account details
BVN/NIN if requested
Request a search for any unclaimed merger/acquisition consideration in his name.
Many investors who failed to update their bank mandates or whose records were incomplete still have such payments lying unclaimed with registrars.
2. The statement he has may be from the old certificate era
You mentioned that the latest document does not bear CSCS and may be from the old system.
If the shares were never dematerialized, then yes, you may need to start with the normal verification process:
Determine whether the shares are already in a CSCS account.
If they are not in CSCS, initiate dematerialization through a licensed stockbroker.
The broker will submit the certificates and ownership details for verification.
Once confirmed, the holdings can be credited into a CSCS account.
However, do not immediately begin dematerialization until you first establish whether a CSCS account already exists in his name.
Since he received a statement in 2018 showing multiple companies, there is a good chance the shares had already been dematerialized and were simply being held in a CSCS account managed through that broker.
Recommended order of action
Obtain all documents he has.
Search for his CSCS account using:
Full name
Former address
CHN (if available)
Request a current holdings statement from the broker or CSCS participant.
Check each company:
Still listed
Merged/acquired
Delisted
Trace any unpaid dividends through the relevant registrars.
Trace any unpaid acquisition proceeds (such as Dangote Flour Mills).
Only proceed with dematerialization if the shares are confirmed to still exist in certificate form.
One thing that stands out is that he says he has never received dividends from any of the six companies. That often suggests either:
His dividend mandate was never registered,
The shares are still in certificate form,
The registrar’s records are incomplete,
Or there are substantial unclaimed dividends and corporate action proceeds waiting to be claimed.
Before any dematerialization exercise, I would first obtain a current shareholding search from the broker and identify the registrars for each company. That search will reveal whether the shares already exist in CSCS and whether bonus shares, rights issues, or corporate actions have increased his holdings over the years.