You will need to rub down the bumper to provide a good key for the primer to stick to. This just takes time and effort with some wet and dry. Use longer strokes and dont concetrate on a small area or you could end up with flat spots. Dont use a machine sander or anything to quicken the process, because you cant control how much you take off.
Once this is done clean the whole bumper using thinners or similar. Onnce totally dry and clean apply a primer which allows for some flex in the plastic, you can get them especially for plastic bumpers. You can get them from most decent car parts shops. An aerosol is fine for primer.
Then I'd repeat this process at least one more time maybe 2. After the final coat of primer rub it down again with some more wet and dry and all should be ready for paint.
No matter how good the painter is applying the paint, if the preparation is not great the paint wont last.
But I'd also have a word with whoever is painting it, show them the bumper and ask what he would do with it to prep it.