There's nothing illegal about her having his SS#. It's what she does with it that could be. I've was separated from my husband for 6 years before I sought a divorce. Being unable to contact him I still needed his SS# to file. Nothing wrong with looking at your own marriage lisence. It's printed...
Call public records and request a copy of your marriage lisence. All you need to do is provide YOUR SS# and for a fee they'll make you a copy. His SS# will be on the lisence.