The grants are paid based on the time the student was in attendance, and are meant to cover the full academic year, which is 1st September to the 31st August. A student is only entitled to grant payments up until the last date of attendance, so afetr the pro-rata an overpayment of £600 does sound about right.
This is covered within Paragraph 38 of The Education (Student Support) Regulations 2009, and was also written into the contracts she signed with the SLC. If she opts to return to education, she will unfortunately be ineligible for any financial support until the overpayment is cleared. The SLC have also been stepping up their legal action of late.