Hi Jay,
It would be good to check with an ESI officer at the nearest ESIC branch.
This is because there had been some discussions earlier within government circles on whether it can be made optional for employees earning above Rs.21,000/- to continue on ESI coverage instead of completely denying them the benefit. The latest we heard was that there had been no final decision and so it stays that those earning above 21K of gross salary monthly would not be covered under ESI.
However for your employee here, if her/his salary hike had occurred during the contribution period from April - Sep'19, causing the gross salary to cross 21K monthly, then you may need to still continue the ESI contributions till the Sep'19 payroll and discontinue the deductions fro Oct'19 onwards. Without ESI contributions, the employee would still be eligible for ESI benefits till June 2020 as part of the benefit cycle ( Accident benefit would not be applicable here). Post June 2020, then the member would be completely out of the ESI coverage.