If the police (& CPS) believe that what she said simply wasn't true, she risks being charged with 'wasting police time'. Such a charge usually leads, upon conviction, to a hefty fine.
If the police (&CPS) believe that what she said was true, and that she's now backing down to try to prevent your boyfriend from being prosecuted, she risks being charged with 'attempting to pervert the course of justice'. That's a very serious charge, which usually results in a prison sentence.
Either way, the CPS can continue to prosecute your boyfriend and still use the original statement in court. (If the events that took place fall into what the CPS regard as 'domestic violence' it's actually their policy, in most circumstances, to do exactly that).
Chris