I actually wouldn't worry too much about coauthorship affecting a student's chances at a postdoc. For that, the advisor should just write in their letter that it's really all the student's work; I would say people don't tend to pay a lot of attention to whether graduating students even have publications, unless there are many of them or they are in very good journals. What matters is what the letters say about the person's research.
Where I have seen this hurt job candidates severely is if they are still coauthoring all their papers with their advisor a couple of years after they graduate and are applying for tenure-track positions. I think people see that and they wonder if the candidate is ever going to be independent.
