There are so many variables that you can't really forecast. -
Any bad character (i.e. previous convictions for assault or dishonesty) which your friend or the victim have makes which is telling the truth more or less believable, if there was any sign of assault on your friend. Even if she did act in self defence, was it proportionate - if there is no mark on her then the injuries to the victim would suggest not. Another variable is even the make up of the magistrates court on the day, and the area you live in - some magistrates courts for example are notorious for letting as many people off with things as they can, (Harringey & South East Northumberland courts for example), but some are the opposite... with so little information I don't think anyone can give you an answer on this I'm afraid.