Just from my experience...
Yes it's painful but not unbearable by any stretch.
I found that tattooing over bony areas is much more painful, so I'd avoid the foot if I was to get another.
I've heard of 'semi permanent' tattoos but never seen or tried them so I don't know how well they work.
I can't stress enough how important it is that you find yourself a reputable tattooist.
There's some helpfull info at the following web site
http://www.nusonline.co.uk/info/health/12471.a spx
I don't want to put your friend off, I have three tattoos, two large ones on my back and an armband, and I've never regretted them. Just make sure its what they want and find a good parlour!
Best wishes and let us know how it goes.