I wish I would've found this thread sooner.
I have both a cat and a ball python. My cat is also able to go outside just like yours.
It's not a dangerous mix! Normally your enclosure will be locked most of the time. The cat can't do anything about that and is unable to get to the snake. If the cat is there, don't get the snake out. If you want to get the snake out, throw the cat (and any other animal) out of the room.
My cat is the most curious little thing and always around me. My brother found her when she was 5 weeks old. Apparently someone tried to get rid of her and threw her in the garbage. We tried to find the owner but weren't able to. I raised her by bottle, taught her to climb trees and have spent nights sleeping in the garden because the cat wanted to go outside but was too afraid to go alone. She has met my ball python (when he was locked in the terrarium) but he was way too boring for her. She looked at him for 2 seconds and then went away. You know your cat better than us. Make specific rules for him but I doubt it will become a problem.
Just make sure to keep the terrarium locked and, if it's not locked, make sure to keep the cat out of the room and there won't be any problems.
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