I have a friend who is going through his divorce and started seeing a new girl. He didn't have a prenup so half of his money goes to his exwife, once the divorce is final. He wants to give some money to his new girlfriend, but because he can't just give it to her in cash to provent from hiding his money, he wants to employ her as his financial advisor. Is this matter ilegal? And if it is, which party would be accused of what?

If she is not providing financial advise this would be considered fraud.

What type of fraud? Who would be in fault?

Look at your question. He is trying to hide money. That's fraud. He would be at fault. She might also be at fault for aiding and abetting.

What is aiding and abetting?

Helping someone commit a crime

First it appears he is hiding the money during the divorce, so if the ex latter finds out about it, she can bring criminal charges for lying to the court and also civil case for hiding the money.

And paying someone for working and they are not actually working is fruad on several levels. Also part of the other issue is that the girl by helping him will become part of the crime for hiding the money to the court.

So what you are saying, you have a friend who is hiding money from his ex wife, he is lying to the court about what money he has, and he has a girlfriend that is an party to this felony

The ex's attorney will not miss this one. Personal and business expenses will be gone over quite thoroughly, I'm sure. Hope the new girl is leaning about his method of operation, she may be next.

Would it be legal, if the guy gave her the money in cash, since they are in relationship?

He can give her cash as a gift but it needs to be open and above board or it will be considered an attempt to hide assets. As a gift it will come from only his 50% once the assets are settled by the courts.