- I have never seen her wearing a dress.
The uncountable noun dress means clothing or clothes. It is used mostly to talk about special kinds of clothing (e.g. national dress, evening dress, battledress etc.). Note that we do not use the article a/an.
- He looks good in evening dress. (NOT — in an evening dress.)
The verb dress can be used to talk about putting clothes on oneself or somebody else. Undress is used for taking clothes off.
- She takes hours to dress in the morning.
- I am going to undress in front of the fire.
- Get dressed in five minutes.
To say what somebody is/was wearing on a particular occasion, we can use the form be dressed in.
- She was dressed in orange pyjamas.