Have the PM go to the PWA Project Center. Then, have the PM click "Go To"
-> "Check in my projects" in order to see a list of all projects currently
checked out to the PM. The PM can then select which projects to "force"
check-in.
In addition to Gina's post, users can be granted Check In permissions to all
projects by adding the "Manage Check In" global permission to the PM security
group.