See Treas. Reg. 1.410(b)-4(b). I think it depends on how many employees are in each allocation group. If the groups tend to include multiple employees in each group, then the "reasonable classifications generally include specified job categories" language indicates one can run an average benefit test. If the groups tend to each have one person in them, then the key provision is "An enumeration of employees by name or other specific criteria having substantially the same effect as an enumeration by name is not considered a reasonable classification." Sounds to me like your client is in the second situation and that the plan has failed to satisfy coverage testing.