I think you're on the right track, letting your brother-in-law try different guns and decide for himself what he prefers.

However, gun selection is actually a pretty small part of home defense/security. Does he know how to make his home a "hardened target" that makes thieves/home invaders decide to go elsewhere? Does he know how to layer security to make it difficult for someone to get in, giving him time to go to a designated safe room? Does he have a safe room? Does he have a means to call 911 from the safe room?

He doesn't have to break the bank to set this up. His safe room could be set up by installing a solid-core door in his bedroom with a deadbolt lock and a charged cell phone. He could make sure exterior doors are solid-core with good deadbolt locks. Motion-detector lights outside his home can be effective. Clearing away bushes and shrubbery from doorways and windows can remove hiding places for would-be thieves/invaders. If you have the means, a security system and/or cameras can be a good idea. Just some examples.

And to echo federali's suggestion, he should also learn about the law and legal implications. Not only what he can and can't do, but also what he should and shouldn't do.

As well as tactics, such as developing a response plan and practicing it occasionally, working out potential lines of fire, etc.

And situational awareness. It's not unheard of for home invaders to follow their victims home.

Some first aid/trauma training would be worth having, too.

He doesn't necessarily have to do this all at once, or even do all of them. Just some suggestions.