I am from Europe myself and moved to the US some years ago after marrying my US husband. I surely can understand how someone would get desperate to stay in the country and ask their friend to marry them. The immigration system was not easy for me and after 9/11 it got even more difficult to get a green card. Luckily we had a good lawyer and got my green card and all. The whole process is very intimidating and took us 2 years. Being married to a US citizen doesn't guarantee that you won't be deported. If they find out that the marriage is fake, you will still be deported and cannot re-enter the country for some years. Trust me, they ask you numerous personal questions about your relationship and spouse to find out if the marriage is real.

I would not marry my friend for the wrong reasons. Not even to prevent deportation. Knowing what they put you through to get legal status, it would be very nerve wrecking for you knowing that the marriage is fake. (My marriage was not fake and we were still nervous wrecks.) They check, double and triple check. There are other ways to prevent deportation. My best advice is to consult with an immigration lawyer and don't break the law. That's what we did and we never looked back.

Hope that helped.