The tax rebate has nothing to do with it.
https://www.gov.uk/guidance/claim-a-grant-through-the-coronavirus-covid-19-self-employment-income-support-scheme
"This scheme will allow you to claim a taxable grant worth 80% of your trading profits up to a maximum of £2,500 a month. It will be available for 3 months, but may be extended."
I assume these are gross trading profits before tax. His books submitted to HMRC should show this figure.
Who can claim
You can claim if you’re a self-employed individual or a member of a partnership and you:
have submitted your Self Assessment tax return for the tax year 2018 to 2019
traded in the tax year 2019 to 2020
are trading when you apply, or would be except for coronavirus
intend to continue to trade in the tax year 2020 to 2021
have lost trading profits due to coronavirus
You will need to confirm to HMRC that your business has been adversely affected by coronavirus. HMRC will as usual use a risk based approach to compliance.
Your trading profits must also be no more than £50,000 and more than half of your total income for either:
the tax year 2018 to 2019
the average of the tax years 2016 to 2017, 2017 to 2018, and 2018 to 2019
If you have not submitted Self Assessment tax returns for all 3 years find out how we will work out your eligibility.
If you have not submitted your Self Assessment tax return for the tax year 2018 to 2019, you must do this by 23 April 2020 or you will not be able to claim. HMRC will review any late returns in the usual way.
HMRC will use data on the 2018 to 2019 tax returns already submitted to identify those eligible.
We will not take into account any changes made to submitted returns after 26 March 2020, when working out your eligibility or amount of the grant.
Find out how HMRC works out your total income and trading profits for the Self-employment Income Support Scheme.
If you have loans covered by the loan charge
If you’re self-employed and have received payment for work or services in the form of a loan or other form of credit covered by the loan charge, you may be able to claim the grant, however your eligibility and average trading profits will be based on either:
the average of the tax years 2016 to 2017 and 2017 to 2018
the tax year 2017 to 2018 if you were not self-employed in the tax year 2016 to 2017
You also do not have to file your 2018 to 2019 Self Assessment tax return by 23 April 2020. You should file by the 30 September 2020.
If you’re a farmer claiming farmers’ averaging relief
If you’re a self-employed farmer claiming farmers’ averaging relief HMRC will use the amount of profit before the impact of the averaging claims to work out:
if you can claim the grant
how much grant you will receive
How much you’ll get
You’ll get a taxable grant based on your average trading profit over the 3 tax years:
2016 to 2017
2017 to 2018
2018 to 2019
To work out the average trading profit we will add together your total trading profits or losses for the 3 tax years then divide by 3.
If you have not submitted Self Assessment tax returns for all 3 years
We will work out your average trading profit based on continuous periods of self-employment, which will be either:
the tax years 2017 to 2018 and 2018 to 2019
the tax year 2018 to 2019 only, even if you were self-employed in the tax year 2016 to 2017
The grant will be 80% of your average trading profit, divided by 12 which will give a monthly amount. We will pay this or up to a maximum of £2,500 a month, whichever is lower.
We’ll pay the grant directly into your bank account, in one instalment.
How to claim
You cannot make a claim yet.
HMRC will aim to contact you by mid May 2020 if you’re eligible for the scheme, and invite you to claim using the GOV.UK online service.