A video from WWDC2019 called App Distribution – From Ad-hoc to Enterpriseexplains the different distribution possibilities for iOS apps.
An interesting one is Custom Apps. According to the video, you can prepare the app as usual on App Store Connect, and associate a Customer ID to it, so that you can distribute it to their employees.
Once the app is available to your customer, they can use an MDM solution to make it available on their devices, or they can use redemption codes, so that the employees, can use their own devices.
I wonder if it can be used by your customer to distribute an app to their clients, via redemption codes.
There is another video, from WWDC2020, called Custom app distribution with Apple Business Manager, but I still need to watch it.