This happens more often than you might think.

At a club where I used to fly there is a chief flight instructor who does this on a regular basis. Apparently the entire board of directors at that club is scared ****less to call him out on it. He did it three times with students two of whom are not rated at all in the space of a single day. He's an accident waiting for a chance to happen.

Even with an appropriately equipped and certified aircraft there's a difference between being instrument rated and instrument proficient.