Is this an example of a participial phrase?
The man in the suit was her former fiance.
No. Your sentence doesn't contain a participial phrase.
Check this site for information about participial phrases.
http://grammar.uoregon.edu/phrases/participialP.html
I didn't write it. It was on a worksheet. But I do have to fix it. I already went to that site and if I were to change it to:
The man wearing the suit was her former fiance.
would that make it a participial phrase?