How Can You Get Child Support For Someone That Uses Another Name At Work?

My sister..who is beyond dumb.....has managed to have a child with a an illegal mexican...she is now filing child support on him and he basically just laughed and said think about it...She has filed under his real nmae however he works under a different name and ss num. He says that he will just tell them that he has no job..which by his real name is true. How can my sister get the court to see that he is under another name at work?