Get the 6GB variant, if your friend has the budget, as it is going to be more future-proof. It depends on what type of games you play though. Some are VRAM-hungry, whereas others not.
But, honestly speaking, the 3GB GTX 1060 card isn't that much of a weak GPU either. It's actually a cut-down variant of the full GTX 1060 6gb CHIP though (slightly slower in performance, but does the job).
Not sure, if you are already aware about this, but this card is not only having 3GB less of VRAM, but it also has lower SPECS, when you compare it with the proper 6GB variant. Actually, a lot Gamers thought they bought a GTX 1060 GPU, which is having only 3GB less of VRAM. But in reality, both these cards are having different specs.
The 3GB SKU has one disabled SM, which actually reduced the core count by roughly 10%. "They also dropped the processing cores to 1152 CUDA cores (128 cores per SM), alongside 8 fewer TMUs, when you compare it with 1280 CUDA cores featured on the 6GB model."
It's a different product though. NVidia's choice to name the card actually confused some buyers into thinking it's just a GTX 1060 with half the VRAM, which is plainly false.