Excellent
By the way, a "Final Warning" is administered to a person under the age of 18 who has admitted an offence. It's more than just a caution or reprimand, because the young person has to have a one-to-one assessment with a Youth Offending Team officer, and between them they will work out why the person is offending, and what needs to be done to stop them continuing to do so. Generally speaking she will have to carry out some small scale restorative action, such as a letter of apology or whatever.
In order to receive a FW for the assault as described by baldycyclist, she must already have been known to the police and have received a reprimand. She now has a police record which will expire (like points on a licence) when she reaches the age of 18. It may however still be disclosable for certain job applications, and will show on a CRB check.
Don't feel guilty that she has got a record because of your incident.... she will already have entered the system for a previous offence. She won't have thought of it as sexual or dangerous at the time, just a good laugh. Fortunately she now has had it all spelled out, and may choose to act with more sense in future.