First, I'm not sure why you're talking about the GBA BIOS. In GBA mode the DS can't run DS programs at all or access the hardware of the DS, let alone write to the firmware. The recovery code is loading a DS program from Slot-2, but it has nothing to do with the GBA other than loading something via slot-2.
The problem with using the 3 in 1 is that he'll need to find something to let him copy some kind of DS code to the device. Even then, it might not have the right kind of code for the system to be able to boot some kind of DS content from there (like a hacked GBAMP or an M3 Perfect, or one of the other slot-2 cards).
Maybe someone with more knowledge will clear this up, but I seem to recall slot-2 devices such as those having special boot code that could distinguish between a GBA and DS right at the start (keeping GBA compatibility but allowing the booting of things in the DS hardware mode), but I doubt the 3-in-1 has it since it doesn't need to have it for what it's used for.
*edit*
Oh, oops, I misread some of what you said. Anyway, I'm pretty sure what I said about why the 3-in-1 wouldn't work is right though.
The problem with using the 3 in 1 is that he'll need to find something to let him copy some kind of DS code to the device. Even then, it might not have the right kind of code for the system to be able to boot some kind of DS content from there (like a hacked GBAMP or an M3 Perfect, or one of the other slot-2 cards).
Maybe someone with more knowledge will clear this up, but I seem to recall slot-2 devices such as those having special boot code that could distinguish between a GBA and DS right at the start (keeping GBA compatibility but allowing the booting of things in the DS hardware mode), but I doubt the 3-in-1 has it since it doesn't need to have it for what it's used for.
*edit*
Oh, oops, I misread some of what you said. Anyway, I'm pretty sure what I said about why the 3-in-1 wouldn't work is right though.








