Is this legal?

evergreen

presumably if he does not have to pay NI contributions, his boss doesn't have to pay employer contributions for him, which means the cost of employing him is much less anyway.

I personally can't see why they should reduce his pay just cause he doesn't pay NI. That affects his net pay not his gross pay. We all have different stopages from our pay, but it doesn't affect our rate of pay.

LittleVoice

Employers still pay full NI for employees over state pension age. There isn't a gain for them there.

By the way, I wasn't suggesting it was right to reduce his pay on the basis he would be paying less NI, only that they might try that as a "creative" justification.