Frankly I'm unimpressed with the disdain. We've all done things we
should have thought through more, and disparaging a past action that he
already regrets is redundant and unproductive.
To the original
poster, as far as advice goes I don't think there is much you can do
outside of checking out laser options or, as previously suggested,
tattooing over it.
The homegrown grown solutions are most likely
completely useless or dangerous. After all, tattoos aren't meant to
come off so any measures that would get them off would be pretty
serious business and not something to be attempted without a
professional.
I recommend avoiding the cheese grater method at all costs.