Get a 64 GB card, fill it up with games and when you get to 50% full, copy contents of card (sysnand & emunand) to your pc.
Put your 128 GB back, restore the emunand back and copy back contents you saved previously.
I can guarantee you 100% that now DevMenu and BBM will work perfectly.
They were right all along, if your 128 GB card has more than 90 GB or so empty space, BBM and DevMenu will not work.
on a side note, what brand is that 128 gb card and how much did you pay for it? a very cheap knockoff might also be the cause of trouble.
since it otherwise works, i guess its formated to fat32 as it should be.
the cia might truly be invalid. did you try more than one? what game were you trying to install? n3ds only games dont work on o3ds.
First of all, you CAN'T have a file bigger than 4096 MB on a FAT32 partition.
Second, a 64 GB card can be had for $30 or less
Third, a 64 GB card will have less lag than a 128 GB
Fourth, if he doesn't play that many games, he's better off with an even smaller card like 32 GB