This is almost as gonzo as my son throwing in multiple red dragons in a level one adventure and then giving me a bunch of silver dragon friends to help out. But he was 7 years old.
This whole scenario smells of prepubescent shenanigans.
The only way I could see salvaging this situation as a DM, short of a hard reset, is to scrap the initially planned adventure. Have the party captured and imprisoned by a rival kingdom who has sufficiently high-level magic users and spys to infiltrate the body guards and find a moment when they can magically transport the characters to a far away dungeon. Since the party themselves are low level, the captors don't have them in a particularly high security prison. But there are wards against divination so the Prince's father's clerics are not going to find them. I would go further and say high-powered clerics in the rival kingdom curse the party with permanent Mind Blank. Prevents them from being found even with a wish spell, but they also get some powerful buffs because of it (they can't be charmed, for example), so they shouldn't complain too much.
Now the adventure becomes escaping from the prison and finding their way through enemy territory to get back to their kingdom.
Another option is, if this is a 5e game, tell your GM to get MCDM's Strongholds & Followers and Kingdoms & Warfare books and run D&D the wargame. The problem here, however, is that the PCs themselves are low level and very squishy. I don't know how they avoid death in a mass combat situation with intelligent and powerful opponents (which are required to make things challenging) without the DM giving them massive amounts of plot armor.
The biggest issue with this scenario is that it seems to be one player whose PC has all the goodies. How is this fun for the other players?
The DM seems to have okay-sured himself into a corner. As you explain it, it doesn't sound like he's enjoying the situation. If that is an accurate assessment of the situation, my best advice is to start over. If the player objects, he can find another game to play in.