Is your mechanic an MB expert? If not, the "pink vs. green" argument is a heated topic among us Benzophiles, but not necessarily to the neophytes.
I had done just that to my 300E after replacing the radiator...only had the green stuff on hand, so I went with that, until I could to a "pink" transfusion the following weekend.
I wouldn't expect any problems with the mix...worst case, the specific gravity and protection properties of the two might alter the limits of the anti-boiling/anti-freezing parameters of either of the compounds in their individual states (in other words, don't drive to Antartica or Death Valley anytime soon).
There are quite a few threads arguing this very topic, from corrosive properties, additives, and long-term wear of seals and aluminum components.
Truth is, these are observations (real or virtual) from many, many miles of use.
In your case, you shouldn't have to worry about it at all. But pick one (or the other) when you do get time.
Forgive your mechanic, but I would agree that he/she should toss in a coolant flush free of charge, as a goodwill offering.