I have a client who ... has a MAC and accesses Kareo..
Recent versions of Apple's Operating System software allow the user to partition the hard disk and run Apple software off of one partition and PC software off of the other. So, without knowing more, one could assume the Mac user is accessing Kareo using PC software like the PC users do. If the Mac is accessing Kareo by using PC software on the Mac, the fact that it is a Mac would be irrelevant.
If you are running business software on a personal (non-mainframe) computer, it's not likely you will ever get very far away from Microsoft - even if you switch to Apple.