Grorge I would not like a #5 rebar ground rod but I think I would have to just talk them out of it or 90-4 them.
250.52(A)(5) doesn't seem to prohibit it.
(b) Electrodes of rods of iron or steel shall be at least 15.87 mm (5/8 in.) in diameter. Stainless steel rods less than 16 mm (5/8 in.) in diameter, nonferrous rods, or their equivalent shall be listed and shall not be less than 13 mm (1/2 in.) in diameter.
There are lots of acorns listed for rebar
