I haven’t used Luna, but first she might want to try the built-in solution: Using AirPlay Mac-to-Mac.
People are more or less familiar with using AirPlay to display their iPhone or iPad screen on a TV, or to use an iPad as a second display for a Mac, but not as many know that you can use AirPlay to use another Mac screen as an additional display.
Have her try this:
- Start both Macs. The next steps assume both Macs are on the same local network, using a recent version of macOS (they don’t have to be exactly the same version), and signed into the same Apple ID.
- On the MacBook Pro, click the Control Center icon in the menu bar, and click Screen Mirroring.
- Select the iMac from the menu. When it connects, make sure the Screen Mirroring menu is set to Use As Separate Display so that it works to extend the MacBook Pro desktop using the iMac display. (If any option starting with Mirror is selected, it’s duplicating the MacBook Pro screen on the iMac screen.)
At this point, the iMac screen should change to the desktop background of the MacBook Pro. If so, it’s now an additional display for the MacBook Pro. Windows can be dragged to it. But before doing anything else, do the next step to avoid confusion…
- Open System Settings > Displays, and adjust the display arrangement as usual (as if it was a typical second display), to set which display is left/right/above/below the other one.
It’s possible that this method might lag a little compared to what she might expect. If she needs more responsiveness, she could try connecting both Macs to the network with an Ethernet cable. Wireless works surprisingly well, I think because Apple might be using a low-latency direct wireless protocol of theirs (which is sort of what Luna does), but of course an Ethernet cable connection can be more robust.
If that works, she might not have to buy Luna which is not cheap. Luna did this wireless external display thing before Apple did, then Apple added this feature to AirPlay in macOS 12..
Edit: It looks like AirPlay might only work up to 4K, so it might not make full use of the 5K iMac display. If she would prefer to use the full 5K iMac display and AirPlay can’t fill it, that would be a point in favor of Luna.
