I'm going out on a limb here by looking at the dive table. Specifically, the June, 2009, NAVSEA "Schedules in the Integrated Air Decompression Table of U.S. Navy Diving Manual, Revision 6: Computation and Estimated Risks of Decompression Sickness". These tables are 10 years newer and more conservative than the '99 USN Tables that many divers still use.
According to this document, a 5-minute dive to bottom depth 250fsw on air alone, requires a 3-minute stop at 20fsw. Ascent time to that stop is 7 min. 40 sec., with a total ascent time 11 min., 20 sec. The deco stop is reduced by 1 minute at the same depth if the diver uses O2 for deco. The ending PG is Z+ and, if the dive is air only, a short chamber ride (or long in-water recompression) with O2 is required.
So the planned dive would have been doable had the participants followed accepted guidelines for that dive. Unfortunately, any plan -- whether it conformed to the table or not -- was blown due to the continued descent of one of the divers, subsequent rescue by teammate, OOA with buddy breathing, and rapid ascent.
That can only be categorized as an accident.