Trading Bitcoin through a trusted friend could be trading Bitcoin through peer2peer with that friend and I do not see a problem with such transaction unless you do not trust that friend. If the friend has a trusted reputation then I do not mind doing business with him because I know him very well that he will not cheat me. If you trade with a stranger, he could scam you and there is no way you can get back your money but your friend knowing you would not scam you because he knows you can always find him and it will spoil the relationship you both have between yourself. If it is only business you guys are having them there would not be a problem but when you start taking investment decisions for your friend by investing for him by buying Bitcoin, then you have gone too far because you will be held responsible if the investment does not get successful
If you have no full confidence doing this p2p even if you say he is your trusted friend, why not use top trading platforms such as binance. I believe, most people are doing p2p because they want to convert their crypto to their own fiat. And now, you will be surprised that most of these top crypto-exchanges are already offering p2p services in so many fiat currencies. You can already see the trustworthiness of the buyer/seller on their star rating/feedback. So at least you already have idea of who to transact with, even if you say, they are strangers over the net.