All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
That the friend of your father works in an investment company does not mean that his wealth is unlawful. A Muslim, in principle, should be rendered free of guilt.
If you do not know that the wealth is unlawful, then the general rule for things is permissibility, and prohibition should not be built only upon probability. So, there is no harm to take the loan from the friend of your father so long as what you have is only doubt.
On the other hand, it is not binding upon your father to write down a deed of that debt, for writing down a deed of the debt is desirable and not obligatory. So, whoever does not write down a deed of the debt is not sinful, but it is preferable for the Muslim to write down a deed of the debt that is due upon him or to him, in compliance with the command of Allaah The Almighty, and to preserve people’s rights.
For further information, see Fatwa 116128.
Allaah Knows best.