It’s a really quick fix. Simply go to the workflow where you are charging the user’s credit card/ subscribing them to a plan and make sure to check “Allow promotion codes”
Also, it’s recommended to use Stripe checkout version 3. I can see that you’re still using version 2. You can upgrade by going to Plugins > Stripe > Stripe Checkout version (change this to v3)
I hope this solves it, your friends should now be able to redeem the coupons you built on your Stripe dashboard 