Is being a jerk infectious?

Can a guys friends influence how you treat a girl? Should girls be worried about who your guy is mating about with? Lets look at it this way..

I once knew a guy, whom we shall call Ross, okay so Ross was a nice guy, he'd never cheated on his girlfriends, and he certainly knew how much it hurt to be cheated on, after three times of being cheated on you'd think he'd have learnt a few lifes lessons.
Okay so lets look at his friends, so two of his closest friends were lets just say, without swearing, man whores, now these guys often had girlfriends, and the amount of times these guys cheated were unbelievable! Ross knew how horrible they treated there girlfriends, and often told me how much he thought they were assholes for doing what they do. Friends being friends though he stuck up for them, backed them up when they needed an alibi for there cheating weekend antics, its what guys like to call a bromance, "bro's before hoes".

So as time went on Ross got more and more close to his friends, or shall I say more and more close to other girls, it makes me think, if his friends werent cheaters, would he have hesitated more about cheating? Would he have been to ashamed of himself to even tell his friends? I mean if I was cheating, I dont think i'd have the guts to tell my friends, they'd be so ashamed, and they'd be the first to tell me what a fool I am. Were as in Ross's case he was in a way praised, gained more respect in the eyes of his friends, at one point he even had a friends mum vouching for him when he wanted to spend the night with another girl and not his girlfriend, (dont even get me started on the up bringing of the guy if his mum is willing to vouch for Ross, the words NO and MORALS spring to mind). So Ross would be telling his girlfriend he was out with his friends, while hooking up with another girl, like its a natural thing to do. If he didnt have the friends to so easily lie for him, if he knew his friends would look down at him for even thinking about being so disrespectful to another girl, would he have cheated?  Or would his girlfriend felt less of fool if she had found out sooner rather than going on for so long being clueless? Does it make it worse knowing she spoke to his friends and they just looked at her like a mug? Would he have lasted a fraction of the time he lied without them?

Okay so thats just like one example, how can I judge on that? Ive not started on the rest of the social group, so he has a bestfriend, now her boyfriend is sleeping with his ex, has been for months, and everyone knows it, even Ross does, has he told her about it? Nope? But why not? Brother code?

So I introduced a friend to Ross, really nice guy, real charmer, any girl would be lucky to have this guy, kind of a softie, he's the kind of guy that would move mountains to get the girl he wanted so badly, and tell her every day how much she meant to him. But now he's changed, I can see this guy going down the same path, all the signs are there...

It just makes me question.. what changes a guy? Is it a social thing? Laugh together about who's heart you broke this week? Who's bed you got into?

Does anyone else worry about who he's hanging around with?