Mixing em-dash and comma

Question

My current understanding of the em-dash is that it can be used in place of parentheses or commas to provide emphasis, e.g.

"Tom, who was a very nice person, walked slowly across the grass."

"Tom (who was a very nice person) walked slowly across the grass."

"Tom — who was a very nice person — walked slowly across the grass."

My colleague has asked me to proof-read some work and has used an em-dash to open the side-comment and a comma to close it. To continue the above example

"Tom — who was a very nice person, walked slowly across the grass."

This seems intuitively wrong to me, and I cannot recall ever seeing this in anything I have read. However, after going through many related questions on this SE and Google, I cannot definitively say that it is incorrect usage and was hoping someone here would be able to clarify.

Answer 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.