@Yagabu 

There is a private cloud solution running on multi platforms (i.e Windows, MacOS, iOS, Android). BUT it only runs on Windows for the host.

https://partners.myquickcloud.com/?pc=fiatlux

If your client doesn't have any plan switching to QB Desktop Windows, the one and only option is using a remote desktop solution. The remote user have to utilize the remote desktop app to "take over" one of the Mac,

https://my.splashtop.com/splashtop_referral/AAHHq6GG344maw

Just my 2 cents.