If somebody is licensed for a Visual Studio Subscription, this implies he is entitled to the following "Subscribers get access to any number of accounts and don't count against your 5 free users."
However, does this imply that if he has one or more customers that he can use Azure DevOps also for the environments/subscriptions of his customers or not? If not, the customer needs to license this (external) person through a basic plan?
Azure DevOps uses Visual Studio subscriptions for those license types. This is assigned by an organization to a user. If the users need access to another organization DevOps's they have two options.
The user can bee added to the second organization via AzureAD B2B (Guest Account), then assign the guest account access to your DevOps.
If the organization does not allow guest accounts, the user can assign a secondary email address to their Visual Studio Subscription profile. DevOps will then check on login if the email address has a subscription, as its a secondary email now it will be detected as a subscriber.