put it on economy and set to cool and then unplug the electrical connection to the heater valve ( I cant think of the correct name of it) and the air will instantly become hot.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual

A better test is to feel the temp of the hoses going into and out of the monovalve, that is the thing that the mech should have been pointing at. One hose carries hot water into the valve the other out, and the lines should not both be hot (actually neither should be really hot) after running only the AC for a while, if they are both hot than it sounds like he was right and its allowing water to flow thru, even a small trickle will "dilute" the cold air in the AC box. This is a fairly common problem, not a lot of AC issues related to a leaking mono valve its usually low gas due to leaks of the freon kind (=$$$)
Another test is clamping the hose off with a small C clamp or even vice grips to block the flow as a diagnostic.

youre stuck with 134 unless you want to pay for a conversion to R12 (used prior to 88) then there is the subject of other hydrocarbon gases but be warned using some of them are considered a rogue act (threads about the alternatives abound).

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Didn't this car use R12 originally?

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2012 E350
2006 Callaway SC560