SILENT MONITOR is a UCD function, not ACD, so she already has the ability. Build a hunt group in which she is the supervisor and the person to be monitored is a member (you don't need to route any calls to the group). I'd suggest giving her a SILENT MONITOR button (321) and a button for the person she'll be monitoring so she'll see when the newbie is on the phone. When newbie's button is lit, press SILENT MONITOR, then newbie's button.
If this is the only person she'll monitor you could make the SILENT MONITOR button a STATION SPEED DIAL instead, and program it with 321 followed by extension for single button operation, but I'd still want a separate button for newbie so supervisor can see when phone is busy.