The PowerCLI 5.1 Release 1 release notes don't explictely mention a PowerCLI Update Manager version.
But since PowerCLI 5.1 Release 1 support vSphere 5 Update 1, I would consider that as implicit support for Update Manager 5.0, and consequently PowerCLI Update Manager 5.0.
I have been using PowerCLI 5.1 Release 1 together with PowerCLI Update Manager 5.0 for some time, and I haven't noticed any issues (which of course doesn't guarantee there are none) :smileygrin: