The relevant question on the ESTA is summarized as follows:
Have you ever been arrested or convicted for an offense or crime involving moral turpitude or a violation related to a controlled substance; or have been arrested or convicted for two or more offenses for which the aggregate sentence to confinement was five years or more; or have been a controlled substance trafficker; or are you seeking entry to engage in criminal or immoral activities?
However, the fine print (click the help link) on the ESTA application says that other factors are involved in answering this question, and to refer to
section 212(a)(2), which states that this moral-turpitude inadmissibility does not apply if:
(II) the maximum penalty possible for the crime of which the alien was convicted (or which the alien admits having committed or of which the acts that the alien admits having committed constituted the essential elements) did not exceed imprisonment for one year and, if the alien was convicted of such crime, the alien was not sentenced to a term of imprisonment in excess of 6 months (regardless of the extent to which the sentence was ultimately executed).
Assuming petty shoplifting is not punishable by more than a year in prison in the UK, then this exception applies, and as I see it, the correct answer to the ESTA question is "no". So from my point of view, your friend has not misrepresented herself on the ESTA, which would be the worst issue from an immigration standpoint.
However, if your friend decides to apply for a visa rather than try to use the visa-waiver program, it remains very important to disclose the violation, as B747-437B has emphasized. The visa application form asks for more detail than does the ESTA.
You may also consider
this link and
this link.
In short, although admissibility is at the discretion of the officer at the port of entry, I don't expect your friend to have any problems. It may be worth contacting a US immigration lawyer to make sure your friend has whatever evidence she needs in case the CBP officer brings up the caution.