Is it permissible to pay the mobile bill with the amount refunded by the person, or must it be paid from one's private account?

It appears the questioner is the original debtor to the company, and his friend is his agent in withdrawing the amount. Therefore, there is no harm for the questioner to use the money and pay the bill with it. However, if the questioner is merely a guarantor for his friend, then his friend is the principal in the debt, and the money given to the questioner obligates him to pay off the debt with it. He is not permitted to dispose of it without his friend's permission, because he is merely an agent for him in the payment.