There are three side to this story: his, hers, and the truth.
His:
He sees this a betrayal of the worst kind, and claims that he could never trust her again. He never wants to see her again and says that it will be a long time before he can ever have a stable relationship after the shock and trauma he has suffered because of her. His best friend, the person he trusted most in the world, has betrayed him, and there is nothing that can be done to fix the treachery that she committed that night.
Hers:
She believes that he his blowing the whole incident completely out of proportion. He was gone, and she decided to have some fun! She claims that it was only one time, and that it will never happen again. She just wanted to have an entertaining night, and it was the only chance she had. She quickly thought out her plan and executed it, and admits that she didn't think about how he would feel when he found out.
The truth:
When her boyfriend left the room for a few minutes, the girl decided to play a prank on her boyfriend by mixing a snack bowl with both skittles and m&ms. She quickly found a bowl and mixed large amounts of m&ms and skittles, and thoroughly mixed them so he would be unable to tell the difference between the two. Afterwards, she sat down and eagerly waited for her boyfriend to return. She wanted to relish in the shocked and confused look on his face after eating a skittle when he was mentally preparing himself for an m&m, or eating both a skittle and an m&m at the same time. This prank-gone-wrong caused a happy relationship to split apart, and the two have not spoken for five years.