Is SQL Server on Mac equally well performing as it is on Windows? My friend decided to learn SQL Server and T-SQL. However, he has a Mac machine.

It is my understanding that SQL Server's primary use case was to be run on Windows.

What potential issues may he run into with SQL Server on Mac?

For example how well does SQL Operations Studio work, does it miss features? Is it reliable?
You can install SQL Server on Mac using docker images. The procedure for that installation can be found here

However, a lot of features still aren't supported when not running on windows (even though Microsoft is working on adding some of them). If you don't need things like Distributed Transaction Coordinator or some of the unsupported features you should be OK.

SQL Server Management Studio will not run on Mac natively, maybe you might have some success using Parallels Desktop.

SQL Server operations studio  will run natively on a Mac.

Bob Ward has a good step by step guide on how to set everything up: Take the SQL Server Mac challenge

SQL Server Operations Studio is still in prerelease status so it's likely to have a lot more bugs than Management Studio and it still lacks a lot of features SSMS has.
If you look at their release notes you will see that the team releases often (sometimes more than one release per month) and you will also see some releases fix regressions in previous releases (which means they broke stuff in one release they had to fix in the next one). You will also note that they add features at a high pace, so trying to list the missing features now isn't going to help you since the list will change again in a few weeks.

If your goal is to start learning SQL Server and T-SQL however I think you should be fine running it on a Mac.