MOH duties vary from bride to bride, its best if you find out whats "traditionally" done and then have a talk with her to find out what she expects from you. Unfortunately I had to fire my original MOH because she just had no clue and kept trying to tell me that I didnt want certain things I did. For example "traditionally" its the MOH's responsibility to throw the bridal shower (and bachelerette party). My original MOH got it in her head somehow I wouldnt want one because she convinced herself I didnt like "girly" things. We were also 3 months before the wedding and she still had made no attempt to find a dress. Fortuantely she took the firing with grace and we are still close. Even though I spelled things out for her and sent newsletters and stuff she still thought all she had to do was show up and walk down the aisle in front of me.
All brides are different, but mainly the MOH is supposed to be the right hand man so to speak and help the bride with all facets of planning the wedding and pre wedding parties (and dont forget the all important toast at the reception)
here is one site (this is not my wedding planning forum...I pm'd you the link for that) that will give you an idea of MOH responsibilities. Just remember these are NOT written in stone and your friend may have different ideas, so make sure to listen to what she tells you she wants you to do
http://www.bridesmaid101.com/maid_of_honor_duties.html