I don't think that the former employer is permitted to consider that these supplemental wages might be used to make COBRA payments. They are essentially severance pay. The former employer has no way to know if an amount equal to these payments will be deductible on the individual's tax return or not. The potential deduction is dependent on you making the insurance payment, not on receiving this supplemental income.
I suspect that many people do not even qualify for this to be a deductible expense. Only medical expenses in excess of 7.5% of AGI appear on Schedule A line 4 and the total deductions on Schedule A need to be greater than the standard deduction for Schedule A to be used (unless the individual has some particular reason to use the lower itemized deductions).
