Actually, with AFP activated, the DiskStation (with Universal Search activated) will let your Macs make use of the remote search index. I think there are some technical limitations to how many folders can be indexed, though, so keep that in mind. (Only 1000 main folders, I think, with unlimited subfolders. May or may not at all be relevant.)
As has already been mentioned by leo, AFP has been depreciated, but if your brother’s company has been using Xserves until now, that‘s likely not a problem for them (yet)?
Also, SMB support on macOS is still not without fault anyway, afaik.
I‘m using my MacBook occasionally to access my company‘s DiskStation via AFP, and search works pretty well. The overall performance of navigating through remote directories feels a bit underwhelming sometimes, but that might be more related to our specific setup. (Not a perfect network setup, no SSD cache for the NAS, etc.)
Interestingly, Acronis offers a software package for Windows Servers to provide AFP file indexing to macOS clients. Very expensive and I have not tested it, but stumbled across it several times as a recommendation:
If for some weird reason (sounds like a primarily macOS-focused envrionment, but who knows), they end up with a Windows Server, then this might be a field to look further into.
This sounds like you can even connect your Macs via SMB, and they will still use the remote universal search index. I cannot test this today, unfortunately.