Usually the Ti is the "2nd revision" of that card, but this time around (10xx series) it is literally just a model between the two as @luke.w has explained. Generally the Ti is different hardware, but it differs from card to card.
I've found that for quick reference, www.gpuboss.com can tell you some specific differences between models if you are looking to compare two models at a glance.
For your information, most graphics cards of the same series will use the same chip, just clocked at different levels and with different features enabled, in addition to differing amounts of VRAM. For example the GTX 960, GTX 970 and GTX 980 all used the same GM204 chip. But 980Ti used the faster GM200 chip (as did the Titan X). So in a way, your friend is not entirely wrong.