Hello Irina. Welcome
Young love can be wonderful and painful.
If she has already told him that she doesn't want to be his girlfriend, then he already knows, but he still wants to be with her. Thirteen is very, very young, when it comes to romance, but love is every bit as emotional.
First of all, if everyone knows that she is his girlfriend, then she should not let everyone else find out that she wants to end it. This must be between the two of them, or he will look silly.
She needs to talk to him again ~ say that she likes him, but that she doesn't feel comfortable being his girlfriend, and wants to be just friends. She must let him know that she didn't want to hurt him, but mustn't say that she felt sorry for him. They can still do things together ~ perhaps in a group which includes you, since you like him. Maybe she could say that she thinks that 13 is too young to be dating.
She must learn from this that she shouldn't be someone's girlfriend unless she likes him. It is less painful to say 'no, but thank you for asking' in the first place, than it is to break up later.
You say that you like him? Do you mean as a boyfriend? Perhaps you could ask him if he would like to go to the cinema, or bowling, or skating, or out for ice-cream? That might cheer him up. Again, perhaps you could go as a group?