There is no such thing as a slight bump. The car weigh nearly 2
tonne, and that weight moving has a lot of force. Yes the bumper
can absorb the force, and visibily the damage is very slight.
but beside the bumper the holders would also be out of
alignment.
You may say it is only a slight bump, but when he takes the car
to the showroom workshop, they will check the chassis for any
distortion regardless. The chassis is part of the safety feature
that should compress and absorb force in a collision, therefore a
slight collision could have the chassis shorten by 5mm. This would
require pulling it back to the manufacturers specification. German
cars are precision made, so any slight distortion would affect
other moving parts.
The BMW still can run, but in time to come there will be noises
here, squeeks here and there as the parts rub against each other
and wear out faster.
Without all that, the bumper, paintwork and workmanship still
cost a bundle.
As driving instructors like to tell their students, Don't hit a
BMW or a Merce.