Leave no woman behind
When your boy has successfully engaged the woman of his fancy, being a wingman becomes less about the approach and more about playing the offensive. It is now your job to prevent her friends from cockblocking your friend.
It's not that women don't want their friends to be happy, they just don't want to feel like left out. Maybe jealously is to blame, but more likely it's just boredom, which is why you now need to get on your One Man Band Man shtick. Be Swizz Beatz.
If you're dealing with a group, be friendly and entertaining. Maybe do double-duty and play wingman for them. The idea here is to have a good time and enjoy their company without creeping. Reassuring them that your friend is a genuinely good guy always helps.