If I understand the issue, the plan limit is 10% of the sum of the pre-tax and Roth deferrals for the year, the excess deferrals are excess amounts that must be refunded. The correction is to make a refund so the total of deferrals remaining in the plan are 10% of compensation. The correction method does not specify any requirement on whether that the refunded excess amounts must reflect proportion of pre-tax and Roth deferrals that were originally made into the plan. The plan also has no guidance.
With the lack of specific guidance, its on the Plan Administrator to decide how the plan will address the situation. The PA needs to keep in mind that this type of operational decision establishes a precedent for future occurrences. One consideration for the PA to keep in mind is the time and effort involved in making the refund. Operationally, refunding from pre-tax first before refunding any Roth is far less complicated than either refunding pro-rata or refunding Roth first. Consider that refunds of Roth get into issues like tax basis accounting, possible taxation of earnings paid from the Roth account, and year of taxation.
If these are not concerns for the PA or the PA is a masochist, then the PA could consider asking the participant to specify how much to refund from each account.