Your girlfriend bought you the bike for Christmas, I think you keep it and ride it. : )

I went to their web site and looked at it, your friend could be right about it not quite fitting you, or maybe not. Hard to say. Your best bet is to take it to a real bike shop and get fitted. You will have to pay for the service, one of the hidden costs of buying over the internet. The biggest issues with frame size are top tube height (can you straddle the frame comfortably) and top tube length (can you reach the handle bars comfortably). If it is way off then you need to move up to their next biggest frame size. They can also put a longer stem on to increase the distance from the saddle to the handle bars.