The 1080ti is superior to the 2070. The 2070 goes for around $500 with a warranty and less performance. The 1080ti used goes around $560-600. So if you can get a good deal on 1080ti..but are you getting a warranty?
In terms of performance, 1080ti is way better than RTX 2070.
However, there's something like warranty for the card. You gain performance, but You loose some of it's warranty time (i assume Your friend has valid warranty for it).
So it's hard decision after all. I went used 1080ti instead of RTX 2070, but i have almost 2y warranty for it.
If the warranty left is less than 6 months or there is no warranty, i'd seriously consider RTX 2070.
If your target is 1080p@60fps get the RTX for sure, you can enable Global Illumination and probably enjoy a nice 60fps in lots of titles with adjusted settings, Plus all that extra ooomph of the 1080ti is wasted on 1080p60. If you are going for higher resolution or higher framerate go with the 1080ti.