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.