I think it depends more on the vet actually assuming the newb can control ascent/descent on some basic level. Most accidents happen when people are complacent not when they are on alert and double checking everything. If the vet is good and watches the diver closely, it will almost certainly go well. When the newb does his 10th deep dive with XYZ stranger buddy, that scenario is far more dangerous IMO. Just enough confidence to overlook things and with a buddy that might not be a "buddy".