You need to have a clear unterstanding about this clients behavior before you'd come to a conclusion whether something is normal or not.
Just as an example below a ping from the EWC shell to my VoWLAN phone model Unify OpenStage WL3.
As you'd see ping reply for # 1-15 is really high - now you'd come to the conclusion that this is a problem... it isn't... the phones is in standby and so the WLAN adapter is in sleep mode.
ping reply #16-21 show the time if the phone is in a active call so the WLAN adapter is not longer in sleep and the reply is about 20ms.
So without further information there is not much we'd read out of it.
I.e. was the client roaming, was he connected to a AP only a meter away from the client.
Maybe a airtrace could give you a better unterstanding what is going on.