I know that if your Adsense gets banned, your channel (google account) associated with that Adsense account (which is the same google account), can never monetize videos again and that channel can never join a network because Youtube prevents it. However, I never heard of an Adsense account getting banned after they are already in a network because networks have their own adsense account.
If your friend can still monetize in that network, once he leaves, he won't be able to monetize with adsense or join another network. His monetization options are permanently disabled. He will have to make a new google account.
You can also make 1 appeal to try to reverse the ban.