Hi @GilesHicks
Welcome to the Sonos Community! And, apologies for the delay.
Yes, your son could AirPlay to a compatible speaker (like Sonos One) and either use it alone, or group-in the Play:1s too.
I would recommend a Beam over Ones though - AirPlay is restricted to stereo when playing to non-Apple devices, and a soundbar is better-equipped to play computer audio anyway, due to it having an input socket. Although AirPlay works to keep the audio and picture in sync, it’s not always successful, so having a direct connection can improve matters.
Please be aware that you cannot connect a Beam directly to a source device with HDMI - it must connect to a device (usually a TV) that supplies HDMI-ARC (or eARC in the case of Beam (Gen 2)), or use a (supplied) optical adaptor. With optical, the source device must have an optical output (which only older versions of Mac Mini have). You may be able to find a macOS-compatible USB sound device that supplies optical output if the Mac Mini your son has does not have the port.
The good news is that if you get a Beam, your son could bond the Play:1s he already has to it, resulting in a 5.0 surround sound system at no additional cost other than the Beam itself.
A Beam (Gen 2) is recommended over a Beam (Gen 1). The second generation supports Atmos, will support Ace, and is slightly improved overall with extra RAM and CPU resources and a newer look. Note that Atmos is not available when using optical.
I recommend a visit to our official eBay stores - all devices are refurbished and come with full warranties!
I hope this helps.