“My friend took his ex-wife and child back into the house as he did not want them living in the streets. He is still paying child support and the checks are being mailed to her but at his address. Is this legal? Please advise and thank you.”

It is legal. If they are still separated - i.e. no intent to be married - then he still has the obligation to support the child. He may be entitled to a modification because he is supporting the child or has more timeshare. However, until a court order is changed, an order is an order. Instead of changing the support order, and if they think the living arrangement is only temporary maybe she would be willing to kick back some support money for household expenses. The facts hint at a potential justification to change custody, but that would be based on what is best for the child.