Yes! As long as a coupon code has not already been previously applied on that subscription (if so, see this guide, here).
Go to the People tab.
Find the end-user's account.
Click on the 'Access' tab.
Click the 3 vertical dots to the right of their active subscription. (If you don't see this option, this means they have already applied a coupon code, see this guide).
Click 'Apply Coupon'.
Select the coupon code to apply.