Constructing objects for external libraries using a model in Laravel
I would implement this as a service in Laravel, and use IOC to resolve my dependencies.

I would make a service, called AdWordsCampaign. This would resolve its dependencies in constructor, namely OAuth2TokenBuilder and AdWordsSessionBuilder, although these two classes are good candidates for services themselves (a factory pattern is most of times is a sign you should abstract creation of class into a service provider).

Then I would implement a method in this service, adWordsSession which takes two parameters: a User instance and $customer_id. Then it has everything it needs to to its job.

I would also implement the reverse case using a service. Semantics are the same.

As for static method, I would avoid them. If you like them, use a Facade. They enable you to use a service anywhere you want.

I strongly suggest to read Laravel docs about services and providers and IOC and Facades. It will give you enough insight about how they work, and where to use them.

As a rule of thumb, whenever you see yourself using new * in your codes, it seems your could use a service/provider and inject it.