His friends like Ron and Hermione would most likely be the ones to help him. They are always there for Harry in difficult situations.
Maybe Dumbledore or some of the teachers at Hogwarts. Since they are responsible for the students' well - being, they would surely come to Harry's aid when they find out he has a heat stroke in the cubbard.
Perhaps Harry was hiding in the cubbard for some reason, like playing a game or trying to avoid someone. But the cubbard was too stuffy and hot, which led to him getting a heat stroke. Maybe he was found later and had to be taken care of by his friends.
Hedwig could be one. She might fly off to get help, like to Hermione or Ron. They could then come up with a plan to rescue Harry from the Dursleys.
Hermione could be a great help. She is very intelligent and resourceful. She could help Harry find evidence against Snape. For instance, if Snape is suspected of cheating in grading, Hermione could use her knowledge of Hogwarts' grading system and records to dig up proof.
His friends could help him. They might drive him to the hospital or call for medical assistance. They could also stay by his side, trying to keep him calm and comfortable while waiting for help to arrive.
Ron would be over the moon for Harry. He'd probably joke around and say something like 'I knew you had it in you, mate!' and then they'd celebrate together with some butterbeer or something from the Hogwarts kitchens.
In some fanfics, there could be a magical creature or spirit that Mundungus has befriended in the past. This creature or spirit, being loyal to him, might try to help him in some way. It could be a house - elf that he once helped, and now the house - elf wants to return the favor. Or it could be a magical bird that can carry messages and tries to find someone to assist Mundungus in his predicament.
Sasuke would also be an interesting choice. Sasuke, with his Sharingan and his quest for power, could use the Heat Heat Fruit to gain an edge over his enemies. He might be able to use the heat to enhance his fire - based jutsu even further or create new and unique attacks.
Maybe Hagrid could help too. He has a special connection with magical creatures, and since Harry is in the form of an owl, Hagrid might be able to understand him better than others. He could provide a safe place for Harry to stay while they figure out how to turn him back, and he might know some old - fashioned remedies or magic that could be useful in this situation.
Hermione could be a likely helper. She's smart and resourceful, and she would likely believe Harry if he told her his reasons for escaping Dumbledore. She could use her vast knowledge of magic and spells to create distractions or to find ways to bypass Dumbledore's security measures.
There could be some old and wise wizards or witches from outside of Hogwarts who come to help. Maybe they heard about Harry's heroics in the battle and feel obligated to use their ancient magic knowledge to save him. And, of course, Ginny would be by his side, using her love and determination to will Harry back to health.