It gives the error at step 17.
Ok question. If he updates to a Soundhax version using a game cart will he be able to use it then? Or will he still be in the same boat?
No, updating the firmware with a game cart isn't the same as updating it when the 3DS was connected online. There's a difference in the two when raising the firmware to version 9.8; the proper path needed for Soundhax needed to be updated with the Nintendo server. Game carts don't provide updates for system titles like Nintendo 3DS Sound.
In the off chance your friend's Old 3DS was updated by game cart, the Soundhax method won't work for him because his copy of Nintendo 3DS Sound wouldn't have received the update with the security hole found between 9.0.0 and 11.3.0. Another way to put it, is that his Nintendo 3DS Sound app itself could really be anywhere from 1.0.0 to 8.1.0 even though the System Setting is stating the 3DS is firmware 9.8 (not fully).
I'm not sure if he can use any of the earlier versions hack methods. At best it either works or doesn't work. At worst, he bricks the 3DS due to mismatches or incompatibilities in the system files.
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If he's been trying with no success at that very last step, I'm afraid your friend's 3DS doesn't have the complete version of the 9.8 firmware.
It's not much of a solace if he was looking for free CFW hacking, but he can update to 11.7.0-40U and install custom firmware by buying a $2 DSiWare game on Nintendo eShop and using the seedminer method.
