I just bought a pound of lactose for $3 or $4 for a sweet stout I have in the fermenter. I don't know the cost of the dry milk, but I can't imagine it being too much cheaper. Also, lactose isn't the same thing as dry milk. Lactose is a sugar derived from milk and dry is literally milk that has been evaporated then dried into a powder.
I've never done a side by side comparison of non-fat dry milk vs. lactose in the same recipe to compare. I'm not saying your mentor is wrong, but in general, I tend to be wary of different things that I hear people say that "you can do" in brewing. Sure, there are a lot of ways to do things, but not all of them are the best way if you're trying to make great beer (I'm not saying that's the case here, just sayin
If you decide to give the dry milk a shot, please post your results, I'd be curious to know how it comes out.