I'm seeing the loss of value for resale to actually be about the same for both the Canon and the Tamron, so in the long run, they both work out about the same. The difference in quality between the two aren't that great to warrant such a large initial outlay, despite the fact the Canon is out of his budget anyway, by a lot.
While a prime will perform better in low light, it's not going to give him any range to work with.
Since these two things in tandem are what he's after, the Tamron is a great option. Saying a prime is superior in low light, while true, doesn't solve his problem.
A prime is great and is a valuable learning experience when it is the only lens on your body. If it's the only lens in your collection that needs to be applied to a multitude of different situations, it's frustrating.
Or if you have something like a 7D, they're
all cross-type.