If you are saying that you want the max value in col G based on the client
in col F then use this formula which must be entered using ctrl+shift+enter
=MAX(IF(F2:F22=123,G2:G22))

Try:

=MAX((A2:A4000=123)*(B2:B4000))

Array entered by holding Crtl & Shift pressed while you press Enter or:

=SUMPRODUCT(MAX((A2:A4000=123)*(B2:B4000)))

Entered with just a simple enter

But lookout for Harlan! (in joke with Harlan)

One quick alternative is to use a pivot table (PT). In a matter of seconds,
the PT can provide both the required uniques listing of the Clients and the
corresponding max values

Select any cell within the source table. Click Data > PivotTable .. Click
Next > Next. In step 3 of the wizard, click Layout. Drag n drop Client within
the ROW area. Drag n drop Value within the DATA area. It'll appear as "Sum of
value". Double click on it, choose "Max" under Summarize by, click OK. Click
Finish. That's it! Hop over to the PT sheet for the results.