Unfortunately, unless she is able to beat Contra, she will never actually see the proposal.
I hear that Contra is epicly difficult. I can't determine whether or not this is true, because I'm crap at video games. It takes a lot of faith to make a subtly modded video game and give it to someone expecting them to see a message hidden at the end -- particularly if it's a difficult one. (If my GF did this to me, I'd never know, because I'd spend a month in the training area and not know how to leave.)