Two points: under the PPP Flexibility Act of 2020, which was signed into law last Friday, your client has a much longer time to spend the money (24 weeks rather than 8 weeks) and in fact the longer period is the default unless the client acts to elect the shorter one, and the 75% compensation threshold for forgiveness has dropped to 60%, so your client may not in fact have extra unused funds now to worry about.
Second - I am concerned with your comment on "prefunding" the PPP money. The PPP rules do clearly include benefits as part of the compensation issue, and there's been argument in our offices about whether the expense must both be incurred and paid during the "forgiveness" period. For example, a discretionary matching contribution for 2019 deferrals doesn't create a legal obligation until the company determines whether it will, in fact, make the contribution and how much that will be, while a safe harbor 3% non-elective for 2019 was incurred when the participants were vested in the contribution, though the deadline for payment (for a calendar year plan) is later this year. I've not updated that research, so this guidance may have been issued already; but if not, until we have guidance that clarifies the "both incurred and paid" issue, I'd proceed with caution. Pre-funding in general is frowned upon for tax reasons, but I think could be problematic in the PPP context as well.