All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If this special care for the rich is permitted by the employer and the employer does not prevent the waiter from taking what is given to him (gratuity), then there is nothing wrong with this act and what results from it; otherwise, it is not permissible for the waiter to act in this manner because it then falls under favoritism for the giver (rich) at the expense of the employer. An employee is not permitted to favor anyone at the expense of the employer without his permission. Also, doing this leads the waiter to be negligent towards the other guests who do not give him gratuity, and this is a breach of trust towards his work duties.
Allah knows best.