bummer. I used to head this off in the interview, even before they made the phone call. Because, if they get to the point of calling your CURRENT employer, they WANT you. Typically, they phone your prior employers ahead of time, but wait on the current employer as everyone knows that will put the word out that you are looking. No one intentionally does that.

Address the issue IN the interview. Sort of a "why I am looking when I have a job" type of thing. Capsulize, and tone will be very important. Have to sort of avoid the "gossip" attitude, the " they are nuts, I am superb" stuff. It will take some drafting at home ahead of time.

Best wishes to him.