It's weird. Dan makes a very thorough prediction that will turn out to be very true following logic based off the facts the story has presented him so far. Arin could reasonably have done the same. However, we do know he's using a walkthrough. Even if spoiler-free, it would probably at some point have to say the name of the mastermind in its instructions. It makes sense people would suspect Arin of peeking ahead. But, with the particular things he got stuck on prior to certain revelations the exploded body's hair and nails, the sudden frequent reminders of those who'd passed would effectively confirm his bias. It is reasonable that he, too, followed his own logical train to the right conclusion, just not via the expected route. The main difference in the Grumps' approaches is that Dan just goes along for the ride, while Arin had a habit throughout the playthrough of trying to stay ahead of the writing.
It could go either way. He may have been spoiled, he may not have been spoiled. Personally--and I'll admit I have no substantial basis for this theory--I think it's more likely that Arin may have come into contact with something innocuous like Danganronpa fanart, and is connecting the dots from the story with dots he just picked up through cultural osmosis. They are 10 years late to this story. But Dan isn't as connected or in proximity to online fandoms as Arin.
So in the end... ¯\_(ツ)_/¯
