The Greater Good (To Do Something Wrong for...)

I hear this saying a lot, "lets do this for the greater good".
So i've struggled with this quite a bit.

For example:
Tina has a Boyfriend named Jesse. Jesse is abusive but Tina is not aware she is being abused. Its obvious to many people Tina is abused.
So Tina's cousin is very concerned. Tina's cousin decides to end the abuse, talking about it with Tina surely is not working, so he concocts a plan, a very elaborate ruse that will "prove" Jesse is cheating on her blatantly and without regard. He will of course deny this, which will fuel the fire even more after more proof trickles showing evidence he met this lady (cleverly photoshopped photographs). In the end, the truth will so far removed from the reality of things, that trust will be lost and perhaps the relationship ends.

Tina is safe from abuse, at the cost of being deceitful. It was however, for the greater good.

Is this acceptable? Id like to know what the majority think of this.

Personally i can see both sides of the story. Doing something very evil to remove an even greater evil is an acceptable cost. My religious side says, don't play the role of Karma, for Karma will bite you 100 times harder down the line.

I understand if someone were to decieve me this way it would be atrocious in my mind, however, depending on the circumstance, i may thank that person for caring so much that they would sacrifice their friendship/relationship with me only to see me be better off because of it.

What do you all think?

[edit on 26-7-2010 by EspyderMan]

To add:
What about governments doing this to their own people? Decieving them for a greater good?

It should prove interesting the responses.