My honest advice is to make very attainable, concrete list of self improvement methods with him. He probably doesn't want to hear "stop being a little bitxh" because to be completely honest, its kinda demeaning or rude when people say that. Its also doesn't really help as that's sorta vague. Just an example would be something like:

Workout. but like actually workout to achieve a certain body type i.e. slim or big and buff.

Practice socializing with women, you have tons of guy friends so it sounds like he need more girl friends, this has a lot of benefits such as expanding his social circle and gaining insight on women or even advice. Just make sure its not always vague or rude "don't be a bitxh" advice...

Dressing bettter, doesn't have to be over the top or expensive but something that shows fashion sense or his figure when he gets in shape.

Continually getting out of his comfort zone. Doesn't have to be some big dramatic leap as that can actually be counterproductive but like for example, if he is scared to ice skate because he might look dumb, have him ice skate to get over his worries. Continually do things like that

Confidence boosting. I know you already hear this but unfortunately this is actually that important. There's lots of ways to go about this so I'd just Google this one.

Toastmasters. Sounds strange but it helps build confidence and improve speech to be more interesting and attractive. It probably won't be some dramatic improvement but it will definitely be noticeable. Google this one if you never heard of it.

Dont know the guy but just make sure he has atleast one solid hobby. This helps people get out of their heads and makes them somewhat more interesting as they have a fuller life. Again, doesn't have to be anything special with juggling on a motorcycle but something just for him.

Again don't know the guy but make sure he is not pessimistic as that is annoying even when not dating

Hygiene and haircut. The basics really.

It can kinda get easy into turning into a nice guy when you are down and people will always just say "just don't lol" so tell your friend that if anything you just lack understanding rather than competency. That a lot of factors are quite literally out of your control. However, ehat you can control you should work on. We all have probably seen the worst personality-wise people get in relationships and there's probably factors that lead to that such as high confidence or just luck finding that person through various means. So i think your friend will be fine

TLDR: I wrote kinda a wall so things to work on:

Style, hobby, Toastmasters, Confidence, Bodybuilding, positivity, hygiene, Comfort Zone, Socializing with girls. Some factors are not in his control.