I know someone who has a corn snake and a rat snake in the same tank

She is planning to sell the corn but even so, surely they were never meant to go together? Apparently the corn is a big boy and keeps pinning the rat snake down. She says it's because he's frisky, but without being able to see it it's hard to know what she means. Could it be aggressive? And even if it isn't it's not exactly fair to the other snake is it?
I would be iffy about putting two corns together, let alone two snakes of different species.

You're right, two different species of snakes should never be housed together, all same-species arguements for cohabitating aside. The sad thing is it's hard to convince people their husbandry is unsafe. Good luck.

It oculd be aggresion, males often try and pin each other down to prove dominance for breedings sake. Also some rat snake species interbreed with corn snakes in the wild, and that is what it sounds like to me, the larger male getting frisky. But your right, you should never co-hab, let alone with different species,