Glue. I am not joking, I think that would be the best, but it would be hard to replace the pedals if he wanted to. Also it requires precision milling for the curve, so it may be hard to do unless you have a rubber sheet in between the aluminium and the steel.

Don't drill holes, but if you decide to do that you could fix the pedals so that they are prevented from moving. Just jam a piece of wood between the pedal and the floor. If you drill then you must tap and glue the screws, DON'T use bolts or nuts from the other side. But... before you do any modification of the car, find out what is allowed and what not. I don't know the UK regulations, and in this respect I have no idea about the Swedish ones either, but here a car is inspected every two years and certain modifications may result in prohibition from driving. I don't know if drilling into the pedals is a critical one, but find out before you do it.