Apologies for the misunderstanding.
You can use SQL Monitor from a remote location, although it will require around 10-15kB/s of bandwidth which is usually a non-issue for internal networks but can be noticeable over slow or constrained VPNs.
As for permissions, you can specify 'builtin\\administrator' with the admin password to monitor a remote machine that is off the domain.
Alternatively, you can create local user accounts with the same name and password on the machine running SQL Monitor and the production server (workgroup style).
It can be helpful in troubleshooting to test any credentials you'd like to use outside of SQL Monitor first. I.e. From the monitoring machine and try to access the admin share / remote registry / WMI of the monitored machine using the desired login details.
I'll review the documentation and see if I can ensure this scenario is covered.