Harry might immediately rush to her side and comfort her. He could put his arm around her and ask her what's wrong in a gentle voice.
Harry could be angry at first, thinking that someone has hurt Ginny. He'd be all protective and start to ask her who he needs to go and have words with. Then, as Ginny starts to explain, he'd calm down and just focus on consoling her.
Perhaps he's feeling isolated. Even though he has friends, he often has to face his battles alone in some ways. Ginny finding him crying could be a moment of vulnerability that she witnesses, and it might be related to his feeling of being alone in the fight against the dark forces.
Often, Harry will offer her a handkerchief or a tissue if there is one available. He might also put his arm around her in a comforting gesture. In 'The Common Room Crisis' fanfic, he does just that and then stays with her, listening intently as she pours out her feelings. He is patient and understanding, which is in line with his character in the original books. He tries to make her laugh or at least smile by sharing a joke or a funny memory. This shows his deep friendship and care for Hermione.
Harry might be initially shocked. He's used to seeing Draco as this tough, arrogant Slytherin. But then, he could feel a bit of pity. He might approach Draco tentatively and ask what's wrong. Harry has a good heart, and despite their past differences, he wouldn't just leave Draco crying. He may even offer some words of comfort or a handkerchief if he has one.
Ginny's reaction could also involve a sense of helplessness at first. But then she'd quickly gather herself and use her resources. For instance, she might call on her friends and family for help. She'd be by Harry's side constantly, using her presence to comfort him and also using her knowledge of magic to try and ease his pain.
She probably comforts him right away. Hermione is a very empathetic person. She might give him a big hug and let him cry on her shoulder.
Ginny might initially be heartbroken. She could retreat into herself, feeling betrayed and confused. In some fanfictions, she may cut off all contact with Harry immediately.
Ginny could be heartbroken. She might initially be in denial, thinking that Harry will change his mind. She has loved Harry for a long time and expected a future together, so this refusal would be a huge blow to her emotions.
Ginny could be heartbroken. She might have expected to marry Harry after all they've been through together. She could initially go into a state of shock and disbelief, not understanding why Harry has refused.
One possible reason could be that she's had an argument with a friend at Hogwarts. Maybe she felt betrayed or misunderstood by someone she trusted. For example, if there were rumors spread about her, it could really hurt her feelings.
She could be a source of comfort. Just by being there, Ginny might make Harry feel less alone.