Unlikely she would even be charged, assuming that he is coming up to 16 and there was no coercion. This is from the CPS guidance for prosecutors:
"Code for Crown Prosecutors - Adult/child defendants
Guidance set out above in relation to sections 5-8 also applies to the child sex offences. The relevant factors that prosecutors should consider are repeated below. The weight to be attached to a particular factor will vary depending on the circumstances of each case. However, in deciding whether it is in the public interest to prosecute a person, prosecutors may exercise more discretion in relation to child sex offences (where the victim is a child aged 13-15) than for offences against children under 13.
Prosecutors should have regard to the following factors:
The age and understanding of the offender. This may include whether the offender has been subjected to any exploitation, coercion, threat, deception, grooming or manipulation by another which has led him or her to commit the offence;
The relevant ages of the parties, i.e. the same or no significant disparity in age;
Whether the complainant entered into sexual activity willingly, i.e. did the complainant understand the nature of his or her actions and that she/he was able to communicate his or her willingness freely;
Parity between the parties in regard to sexual, physical, emotional and educational development;
The relationship between the parties, its nature and duration and whether this represents a genuine transitory phase of adolescent development;
Whether there is any element of exploitation, coercion, threat, deception, grooming or manipulation in the relationship;
The nature of the activity e.g. penetrative or non-penetrative activity;
What is in the best interests and welfare of the complainant; and
What is in the best interests and welfare of the defendant.
In summary, where a defendant, for example, is exploitative, or coercive, or much older than the victim, the balance may be in favour of prosecution, whereas if the sexual activity is truly of the victim's own free will the balance may not be in the public interest to prosecute.
In addition, it is not in the public interest to prosecute children who are of the same or similar age and understanding that engage in sexual activity, where the activity is truly consensual for both parties and there are no aggravating features, such as coercion or corruption. In such cases, protection will normally be best achieved by providing education for the children and young people and providing them and their families with access to advisory and counselling services. This is the intention of Parliament."