I know absolutely nothing about homebrew on 3ds, and I have an old 3ds xl on 11.1.0-34U and I was wondering if there is a method to edit the save files of Pokemon Yellow Virtual console. If you know, please explain it so that it is very clear.

If he buys an entrypoint then he could just use the 3dsx version of JKSM.you either hardmod your console or use another CFW 2/3DS to do the DSiWare Downgrade to your system.
then you can edit your pokemon save with JK save manager and PKHeX
Pretty sure the xmls go inside 3ds>JKSV not just on the root. Try that. If it doesn't work then no harm done.Every time I try to use JKSM on Pokemon Yellow, it just gives me black and red screens. I copied the mmap folder of xmls to the root of my microsd card, too. What am I doing wrong?
I remember them only working with CFW too but IIRC that was because the only save managers that COULD dump the data were .CIA only. JKSM now has a supported .3dsx so it should work fine with the xml files in the right place.I remember that the GB pokemon VC saves could only be accessed with CFW. Which is why PHbankGB is .cia only