I'd suggest that there are actually two VERY WEAK links in his plans.

The weakest is the human ability to come up with anything out of their own mind or body with more than 160 bits of entropy.  He may think that his passphrase is going to be "amazing", and that if it ever shows up in a rainbow table then "one will ever be able to support a brainwallet ever again once he's shared his compromised passphrase on the internets", but I suspect that he's overconfident.

The next weak link, after his not so random passphrase, is his memory.

And to top it all off, he refuses to write it down to store it anywhere, but he plans to "share the brainwallet with my trusted family members in case anything happens to me".  Almost certainly one of those "trusted family members" will be concerned that they might forget it, so it will be written down somewhere and he won't have control of the storage and safekeeping of that paper wallet.  Wouldn't it be better to secure the paper wallet yourself and then share with trusted family members the information on how to access it if anything happens to you?