Interest will not accrue on the 6-year amount, as it only accrues on loans that have been disbursed.
A $6,000 loan will accrue about $400 in interest in 1 year. So at the end of 6 years, a breakdown of the interest from oldest loan to newest would look something like this:
Loan 1: $2,448 (intrest accrues for 6 years)
Loan 2: $2,040 (interest accrues for 5 years)
Loan 3: $1,632 (interest accrues for 4 years) etc.
Loan 4: $1,224
Loan 5: $816
Loan 6: $408
For a total of $8,568.
Keep in mind that at the end of her grace period, this interest will capitalize, so if possible, it's always good to pay off some of the loans while in school.
Sallie Mae has a good accrued interest calculator if it helps: https://www.collegeanswer.com/tools/acc ... fault.aspx