Does your wife wear glasses? Some headsets may be less comfortable than others when users have glasses on.
What's your ideal budget? Inexpensive to one person isn't necessarily inexpensive to the next.
Wireless or wired, or is either fine?
Does she have any particular style of headphones she's liked or more particularly not liked before? For example she may or may not have ever had headphones that completely wrap around her entire ear, so a headset with earphones in that style would be a new experience (which could be good or bad!).