As most will be aware, the MOT test is essentially only a valid outcome at the time of the test. At any other time components may be within MOT tolerances but at the time of the test not and vice versa. On top of this the extent of testing that can be done without carrying out a full MOT test is invariably limited. The obvious, in terms of tyre tread depth, windscreen damage, etc may be assessed but other items such as corrosion, exhaust emissions etc., are not so straightforward.
The client is quite right that the notion of a service referred to as a pre MOT check is in fact quite dangerous, even when carried out in good faith.