Mixing em-dash and comma

Solution 1:

The Punctuation Guide says:

A pair of em dashes can be used in place of commas to enhance readability. Note, however, that dashes are always more emphatic than commas.

And yet, when the car was finally delivered—nearly three months after it was ordered—she decided she no longer wanted it, leaving the dealer with an oddly equipped car that would be difficult to sell.

From the explanation above (with the emphasis on a pair,) I would say that your colleague's sentence is a definite no-no.

The Grammar Girl confirms this, she says:

It’s also important that when you set off a phrase using em-dashes that you used one em-dash immediately after the noun the phrase is describing and one immediately after the phrase. Don’t replace the second em-dash (as some tend to) with a comma or semicolon.