Hi Kumar
I think that you should use planning strategy 70 to plan at component level. That way, the BOM changes will not affect the raw material procurement.
Otherwise, you can suggest your customer to create pre-defined lunches, with different material numbers. For example:
Lunch1 = Chicken, rice, salad
Lunch2 = Meat, onions, eggs
That way, you don't need to change the BOM so often.
One last remark is that frequent BOM changes will probably lead to performance issues during the BOM explosion and MRP run. Therefore, you must think on a periodic BOM archiving to avoid huge BOM tables.
Regards
Caetano