Actually, if the thread was about benefit fraud, there would be quite a few supporters.
I don't think anyone here is saying what he did was right. What most of us are doing is trying to help the OP out of the situation she is in (legally, I must add). She knows what he did was wrong, he knows what he did was wrong. Condemnation from a few Mumsnetters isn't going to add to what they already know and feel.
I have some sympathy for the situation. I can imagine that it is quite easy for a man who is young and single, from a country that doesn't have the same regard for the law, to basically drift into the situation he is in now. As the OP said, he didn't count on falling in love, which changed everything. I'm sure for the past several years, he has been living always looking over his shoulder, knowing that the day would come. Well, it has come now.
I am not shocked about the NI number. He wanted to work, didn't want to do cash under the table jobs, so needed an NIN. He did what was, I imagine, pretty normal for him - he bought one. He is from Brazil, remember, where you pay for everything, even if it is legal (you often have to give backhanders to officials). We don't know the full circumstances of the NIN - he may have had people helping him who told him this was how you do it. That wouldn't have been that unusual to him.
I imagine that once he decides to return to Brazil and set in motion the EC process, he will be mightily relieved. It will be a tough process for him, but it can't be easy living as an II either.