You can invest and trade most of it, but maybe keep 1 bitcoin fot yourself and purchase things such as Amazon cards
He cannot buy anything from that because he mentioned that he taken 3 btc as a loan and would have to repay more that 4btc back to him so cannot waste 1btc like that.Still i dont like this idea at all because its a really risky thing to do and can result in loosing even those 3btc as well.