You can do it without issues...
The Verizon iPhone 6 is the same model as the T-mobile version. It is unlocked by default and will give you the same exact VoLTE/WiFI Calling/LTE on bands 2&4 experience as though you had a T-mobile iPhone 6
one warning: If your friend ditches Verizon and refuses to pay off the ETF, Verizon may blacklist the device and may pass that blacklist to T-mobile, which means T-mobile might block you from using that device
also, your friend might be an idiot if he's on a non-grandfathered Mobile Share/Value plan... For those plans, you get a $15(<10GB) or $25(>10GB) per line per month discount for EDGE or off-contract... Those discounts are void if you sign a 2-year contract.
2-year contract = $199 down + $25/month * 24 months = $799 that you are paying for a $649 iPhone 6. It might make more sense for the $15/month discount (phone costs $559)