Hi Mark,
I had the same issue, but the workaround i did was:
Once the HR selects the User i display another form and using workflow send the respective information. & at the end i have a dropdown button to ask the HR, are you sure to terminate the user (yes/no)?
the confirmation dropdown is marked as mandatory so the HR will have to select yes/no
And then using workflow i perform rest of the operation.
But the catch is that if they do select the wrong user, (they will have to relaunch the form) & i configured using predefined search (so they were able to search using display name and email)
..
Linking the thread for more information if you want to display the information.