They must have been done by you, unless you're sharing your account details with your friend, which is against the T&C's anyway.
Not necessarily, his friend might have come round and used his PS3 with his log in details stored on it without Soulfullassassins knowledge.
I've get 32 platinum's myself including some long grinding and hard ones such as Resistance 2, GTA 4, Dead Space 2 and MotorStorm: Pacific Rift and every single one of my trophies have been earned by me without cheating/hacking, check my profile out if you want to.
The thing is, he has several Socom: Confrontation ones which were achieved within a couple of minutes. Some of those are really hard to get and there was a hack out that let you do this.
E-bay is a cheats friend lol. You can get trophys ( like he has with Socom ) for a few quid. Crazy people do it, but there ya go.
Pretty sad really. Worse thing they did was bring trophies into the mix, what happened to just playing because you like it and not trying to outdo your mates for a trophy.
I'm not even bothered about getting them. In fact I don't even attempt to get them in-game.