Do you realize how long that would take? Your alphabet is 62 characters, and your string length is 12. 12
62 =~ 8.11x10
66. That's an 8 with 66 0's after it. Let's say it takes 1/1,000,000,000,000 of a second (1/trillianth of a second, clearly unrealistically fast) to check a single value. To check 8x10
66 values, that would take:
8x10
54 seconds
2x10
47 years
That's right, it would take about 250,000,000,000,000,000,000,000,000,000,000,000,000,000,000 years to guess every combination. I don't think your friend should bother waiting that long to get his items back, since the universe probably won't be alive that long.
As was said, you'd have more luck with a dictionary file. I have a 125-mb dictionary that has every word in every language. If you take those, append/prepend numbers, replace i and o and e with 1 and 0 and 3 in every combination, you'll have a pretty complete set. It would still probably takes months to go through all that, but at least the Universe will still be around.