I agree with Leathermang - these automatic climate control systems in the W123 are complicated and know weak points of the car. There is no evidence your mechanic did anything wrong - the failure could be totally unrelated to the work he did.

I've spent $600 on ONE part for a Mercedes A/C system - they are very expensive and there are lots of parts that can go bad.

$550 is nothing for an A/C repair these days, especially in California$$$$