Saying no to them begging for treats is simply not giving them treats. If your friend gives into cage rattling, nipping, etc., they are only reinforcing that level of begging for the future. The way to say no to treats is to ignore them and not give them.
If the diet is objective reduction of food (ex. less pellets and treats), you can just make that shift right away.
If the diet is changing to healthier pellets and/or vegetables, it needs to be a slow transition as to not upset the rabbit’s stomach.
In tandem with a diet often needs to be increased exercise time as well. Your friend should reflect on how much roam space the rabbit has and if that space is accessible during the rabbit’s active times (early morning, late evening). A better diet will not do much if the rabbit has a sedentary life.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.