I have the Canon 17-55 f/2.8 and I would recommend it if he has the money and don't mind some extra weight, also I must agree the image stabilizer is very nice in low light condition shooting at 50mm. I also think the Canon is faster at autofocus which can be a big plus.
That being said I have no experience with the Tamron but it seems very sharp and if I had to buy my own gear I would have chosen the Tamron (me and my dad share most of the gear).
Search google for some reviews and comparisons between the Canon and Tamron.