Did she overstay in the U.S.? If not, she should remain eligible to enter under the visa waiver program.
The cocaine charge, though, could impede reentry. We know none of the facts, so she would likely benefit from consulting an immigration lawyer. Also, given that she previously evidenced intent to immigrate, the immigration officer may want to see strong evidence of her ties to the U.K. to support her claim that she intends to return.

