Tone of voice generally used to ask people for favors. Overly nice and soft toned. Normally not seen as a good thing.
Rosie uses her waiter voice to convince me to do her homework,
A Dater Waiter is friend who dates the person who broke up with you, and refuses (or can't/won't find) to date anyone else. This could be from to make you mad, trolling, or they had feeling for them before.
girl: i'm dumping you
boy: ok :(
friend: wanna date me instead
girl: sure
boy: you are such a Dater Waiter, man