Hi Anne
The rights in a user guide are most likely going to lie in copyright as protected works, probably a combination of literary and artistic works.
The rights of those will accrue to the author absent some other agreement.
If the author was an employee and the guide was created in the course of employment the owner will be the employer.
If the author was a contractor, the rights in most ordinary cases, accrue to the contractor unless otherwise agreed.
It does not matter that the author does not hold IP rights in the actual software. Issues might arise from the use of screenshots etc which may be of questionable ownership if not created by the author of the guide. But otherwise, the ownership in the guide is pretty much a distinct issue to the ownership in the software.