Men should always be unaccompanied while shopping for their own clothing, in my opinion. On the best of days, shopping companions merely distract sellers who should be giving their attention entirely to you.
This advice applies tenfold to men who are in a relationship with a woman. In my experience, members of the opposite sex believe that XX chromosones impart innate expertise about men's clothing. And no man should be forced to listen to advice from someone who might cause her companion to dress as actor Robert Downey did at the Oscar ceremonies recently.
It is after all difficult, if not impossible, for a man to make considered decisions about clothing while accompanied by a loved one who is convinced that she knows better than he what looks good on him, and which places he should patronize. For nothing can come of that but black suits, or worse.
Always shop alone.