I like to think about allies and fake allies in a movie as a true friend and an fake friend.
What is most controversial about the unveiling of a fake friend is that, before that moment, he/she was the person you trusted the most.
In real life we should not judge people by a single occasion but trace a path along shared experiences. We all deserve not to be judge by a random situation: friendship it's a matter of resolution and understanding.
Nevertheless, we might have someone on our side always planning to damage us. Reasons can be various and we can't describe all of them, but the common trait is that they are not able to celebrate our successes with us, but they are very good at pointing out difficulties at our worst moment. If you have ever experience that, you still have a friend.
A fake ally is a totally different subject. Is someone faking being at your side, but constantly planning being on the other one. Therefore, he's not acting to sustain you, but to make you fail even bigger. How can you spot a fake ally or fake friend in your life and in a story?
First, he has a strong interest in everything you do, but don't be fooled: he has a personal plan and a goal to pursue, a fake friend is there to challenge the hero and let the audience see what's below the surface.
Second, to be on your side he will do whatever is possible to be perfect for your and to become essential in your life.
And last but not least, he won't really be happy for any accomplish you can get. Any, because they will be a distraction from his goal to reach.
This Brit picture shows a fake ally as a visible red phone booth
on the side of your walking line with a clear message : PULL.
It will pull you back from your greatest accomplishment and will pull you down with painful opinion about your life. There's no reason to trust such a noisy presence: keep going, don't stop, don't let fake friends ruin your big moments in life.
The difference between fake friends in a story and in real life is that in the first case they are designed by the writer to create a situation that will enhance the hero personality and moral rules; they exit to put him to the test, because making decision, and being active in a story will shape the character towards his goal.
I like to think that people in my life that failed me were on my path just to make me stronger and wiser. This makes me feel powerful because I realize every time that power is in my hand, not in theirs. I'm the one writing my own story.