Hmmmm. Perhaps you could encourage them to think of a few things they like about
themselves? It's hard to be liked by other people if you don't like yourself.
Conversely, you could very nicely and gently explain what this person is doing to make him/herself so disliked. Sometimes people really do have no idea what they're doing to antagonise others, and it can really help them to have some honest, caring feedback.
Sometimes people are unusual, real individuals, and it can take them a long time to find a niche where there are folks who accept and like them for who they are. For instance, a tween who is a lot more mature than her peers may find that she makes friends more easily when she is older, as the people around her gradually become more mature. It may also be that someone is simply in the wrong school / social group / town, and that they may be happier if they move. For instance, someone who is bad at sport will probably not feel happy hanging around with a lot of athletic people, but might feel happier in a drama society or book group.
Not being liked doesn't necessarily mean that a person is unlikeable - it can just mean that they are in the wrong place, at the wrong time, with the wrong people.
Ivy.