do (one) a service
To help or aid one. Sharon did me a service by getting that jammed paper out of the printer.
do (one) no service
To do something that hinders or is detrimental for one. You do your kids no service by catering to their every whim.
do somebody a/no ˈservice
(formal) do something that is helpful/not helpful to somebody: She was doing herself no service by remaining silent.