She'd do it so that she doesn't run the risk of having to sit next to someone she finds objectionable for several hours.
Yes, it can be done, but you need to be clear with the airline what you are doing - if they just see two seats in the same name then they may well cancel both of them. Obviously you have to work with them to make sure that the two seats you have are reserved adjacent to each other, otherwise it somewhat defeats the object!
How well this works varies from airline to airline according to reports.
If I were you I would ask the question on the forum for the specific airline you are thinking of flying over at
http://www.flyertalk.com
