Sounds like you found a reference to a SARSEP (Salary Reduction SEP). You haven't been able to set one of those up since TRA 86 and I don't if any one still has them.

Or the SEP has an employer contribution part plus an employee IRA part (I've never seen this.

So, for a SEP there is no over 50 catch up. The max is 25% of net self employment income.

Now SIMPLE and 401(k) do have over 50 catch up provisions.

Have your client contribute $6,000 to an IRA if that's the issue.