The first thing her solicitor/barrister will probably need to do is to examine the evidence relating to the injuries sustained by the guy. A 'GBH' charge (irrespective of whether it's 'with intent' or not) requires a particularly serious level of injury in order to obtain a conviction. From the CPS website:
"Examples of what would usually amount to really serious harm include:
injury resulting in permanent disability, loss of sensory function or visible disfigurement;
broken or displaced limbs or bones, including fractured skull, compound fractures, broken cheek bone, jaw, ribs, etc;
injuries which cause substantial loss of blood, usually necessitating a transfusion or result in lengthy treatment or incapacity;
serious psychiatric injury. As with assault occasioning actual bodily harm, appropriate expert evidence is essential to prove the injury"
So, unless the guy needed a blood transfusion or suffered a long-term disability, a charge of ABH might be more appropriate.
It would also be necessary to consider whether a plea of 'self defence' can be used. The basic principle of law is derived from a 1971 precedent: "It is both good law and good sense that a man who is attacked may defend himself. It is both good law and good sense that he may do, but only do, what is reasonably necessary."
However the law also recognises that, when being attacked, it's unlikely that someone will be able to step back from the situation and carefully assess what is reasonable. From the same 1971 ruling: "If there has been an attack so that self defence is reasonably necessary, it will be recognised that a person defending himself cannot weigh to a nicety the exact measure of his defensive action. If the jury thought that that in a moment of unexpected anguish a person attacked had only done what he honestly and instinctively thought necessary, that would be the most potent evidence that only reasonable defensive action had been taken ..."
So, as I see it, it's unlikely that the CPS has got a good case for a Section 18 (or Section 20) GBH conviction. There's also some doubt as to whether they could obtain a conviction for 'ABH' if a plea of 'not guilty' is entered. (Your daughter needs to take the advice of her solicitor/barrister with regard to deciding whether to plead 'not guilty' on the grounds of self defence. Most attempts to use 'self defence' in such a way are usually doomed to failure but your daughter's case may well be an exception).
If your daughter was convicted (or either ABH or GBH with intent) she would have to sentenced as minor (because it's her age at the time of the incident, rather than her current age, that's applicable). A sentence for GBH with intent (even for a juvenile) is almost always custodial. (The minimum sentence for an adult offender is 3 years imprisonment, except under very unusual circumstances). An ABH conviction involving the use of a knife could also well result in a custodial sentence but non-custodial sentences are also available to the court.
Your daughter needs to seek the advice of her solicitor (and ideally insist upon a meeting with the barrister who will be representing her at Crown Court also present). Very careful consideration needs to be given to whether she should plead:
(a) guilty to GBH with intent (although I can't see it likely that such an option would be advised) ; or
(b) not guilty of GBH with intent but guilty of ABH (which the CPS might accept before the case gets to court anyway); or
(c) not guilty of any form assault (on the grounds of self defence).
One last thought:
The CPS might be forced to drop the case anyway if their star witness is on the run himself!