I made a simple follower, then placed her inside Jorrvaskr. She's not a duplicate, but new. She does everything a follower is supposed to do. Here if anyone was curious http://steamcommunity.com/sharedfiles/filedetails/?id=115475173. I have Apachii and Bella's installed. No issues whatsoever.

OK, I send the mod to my dad. He also has Apachii and Bella's installed. He does a lot of Companion side quests, so I thought that be a good place to put her.

As soon as he tries to run the game with Alessa's box checked, instant CTD. No messages.

I'm level 81, and he's at level 50--both machines are Win7x64, with 670 gtx's, etc. He's 2000 miles away, so I can't look to make sure everything is in place. He says all the boxes are checked, and Apachii and Bellas are way up on the load list--Alessa is set to load last. He tried her with Apachii and Bella's unchecked. Same result. She's the guilty party.

Besides her usual Warlock goodness, I added a spell to her list I had made some time ago called FlameParalyze. I unchecked it from my data files, and restarted the game. Alessa still had the spell in battle, so I assumed it was good to go. While in battle, she uses that spell as her first choice, so I'd like to keep it available in his mod.

I've made him a couple spells, and a summonable Dwarven Sphere--no problems with those...just Alessa.

Sooo, what error have I found myself in the middle of? Is it the custom spell messing things up? Does it need to be added to his data files as it's own mod? Or, removed from her spell list altogether?
Or, is it something else entirely?
Thanks for any input.

Sounds like your mod is dependent on a master file that your father doesn't have. What files have you set as masters for your mod?