We can get into the most trouble in the safer sex department with our buddies.
There are definitely advantages to knowing our sex partners such as increased knowledge around our buddy's sexual health, the general number of sex partners he has, and a better chance he would disclose an HIV or STD infection to you than an anonymous sex partner. These can be good things.
The unfortunate reality is that we have an implied trust with our friends, we don't ask them the same questions we would with a stranger, or take the same precautions. Just because he's your friend doesn't mean he automatically has the same HIV status as you. Have you asked? Has he tested (and cleared his window period)? How do we know what we know?
The danger is less about whether he's lying about his status... the danger is more that he may not know his status as well as he thinks. Unfortunately, guys are most infectious when they are newly infected. This also happens to be during the the window period (3 months) when a guy can test negative on an HIV test, but actually be infected.
A good rule of thumb: if he's barebacking with me, he's probably barebacking with others.