How do you tell a girl that her friends suck? You don't.
No seriously, there is no way that telling your girl that you don't approve of her friends will ever end well for you. It may sound like a good idea at the time to tell her that her friends suck, but you can't ever utter that phrase. You can't even utter anything remotely close to that phrase. And here's why:
I know it may be hard for some of you to realize this, but your significant other's friends have way more influence in your relationship than they should. I'd even go as far as to say that your girlfriend's/wife's friends matter more to your relationship than you do. What I mean by that is that you must keep your girlfriend's friends happy at all times, and they have to be happy that she is with you at all times, or all hell will break loose figuratively and literally. Her friends will go from supporting you to undercutting you at every possible opportunity, to the breaking point where she usually has to make a decision between you and her friends. You never want to end up in this scenario, so again you can't ever say that her friends suck.
The most common way that your significant other's friends will try to intervene in your relationship is that they try to apply things from their dating and personal life into the way that you interact with their friend. That failed dinner date that your girl's friend just had will somehow become a topic of your relationship, and you will have to address whether you would have acted the same way or a different way. Regardless of what your answer is, you will be judged ruthlessly. This scenario is exponentially increased if this discussion occurs in the presence of both your girl and her friend, as they will try to dispute any instances of logic you may use to rationalize your argument. Reacting logically to a situation with a woman is like voluntarily drinking Clorox bleach. It never ends well.
I had a situation in my previous relationship where while hanging out with my girlfriend and other friends, one of her friends interjected that I couldn't have any single female friends because it's impossible for men and women to be platonic friends. Her reasoning for this was that every man who's ever claimed to be her friend has wanted to sleep with her, so therefore I must behave like every man she's come in contact with. The problem wasn't the interjection, or my defense of the contrary that men and women can be friends on a platonic level. No, the bigger issue was that someone with this level of close-minded world views was someone that my ex-girlfriend trusted for advice, and that this was negatively affecting her ability to think for herself in muddied situations such as these. Men and women can be platonic friends. It's very naive to think anything other than that, just like it's possible to be amicable to coworkers without being friends with them or trying to sleep with them. I don't have to have ulterior motives just to be nice to someone.
I won't even get into the resolution of this situation because like I said previously it involves my ex-girlfriend. I just want to state that sometimes it's best to see these scenarios occurring like a quarterback reading a defense and react accordingly. And be on the lookout for your significant other's friends rushing you from the blindside while you focus on reading the secondary.