Did you say that the caution was 2 years ago? if so you've only got 3 months from the date of the caution to take it to the court of administration in london to get it rescinded, or you've got 12 months from the date of the caution to complain to the police station in which the caution was given in order to get the caution rescinded, after this you can't do anything.
IF you had taken this to a solicitor within this period you could've applied for the caution to be rescinded based on the grounds that the police didn't follow procedure when giving the caution. Believe me i've had a caution, did this and had it rescinded simply based on the police not following procedure. For a caution to be given you must admit the offence, the officer must explain the implications of a caution, eg not being able to get a job working with children, vulnerables (mental health patients) etc. What you must remember is, although they are a police officer, unfortunately for all of us they don't know the law, and hundreds upon thousands of young people are being criminalised falsely and getting away with it simply because the police aren't following procedure and simply doing what the hell they like. the only advice i can give you is continue in a course that doesn't result in working with any categories that you need a CRB check for, get a job and then after 5 years the caution should be spent on your girlfriends record and won't affect her applying for jobs after that.