Is it permissible for a man who stopped messaging a girl he loved, seeking Allah's pleasure, to ask Allah to unite him with her in Paradise, even if they did not marry in this world?

There is no objection to a man supplicating that Allah Almighty unite him in Paradise with a woman he did not marry in this world. Ibn Taymiyyah mentioned that if a man loved a woman in this world but did not marry her, and he gave her dowry in charity and asked Allah Almighty for her to be his wife in the Hereafter, then this is hoped for him from Allah.