One instance of abandoning a friend should not change a characters alignment, especially not a decision that was reconsidered. One action should very rarely ever change a characters alignment, with only extreme cases being the exception.
Abandoning a friend isn't a Lawful Good thing to do (I'd say the deliberate abandonment of a friend to death is both Chaotic and Evil), and should end up in a loss of Paladinhood, but not a change in alignment for just one incident.
Now, twice, on the other hand, I'd have a very good chance of taking a character straight to True Neutral (or Chaotic Neutral.) Three times, definitely.
Abandoning a Good friend to death is a very bad thing, but one strike to and alignment change is too harsh, in my opinion. People have very bad days, get angry, and make bad decisions sometimes. But twice is enough of a trend with something of that magnitude to justify an alignment change, I think.
Remember that forgiveness is an important part of being Good. There should always be the possibility of forgiveness for all but the most absolutely heinous of crimes. (deliberately destroying a friends soul out of malice, for example, would be something of that magnitude.)