Check if he/she is in the possession of a valid temporary “A” card.
What else do I have to do, besides the normal obligations applying to every employee/employer relationship ?
Nothing, there is an exceptional temporary measure for those Ukrainian national, that exempts you from requesting a Single Permit.
There is no exception to the normal rules in that case: he/she will have to hold a valid professional card.
Nothing has changed: a recognition request shall still have to be filed in advance.