George joined Boro from Doncaster Rovers in July 2012 for an undisclosed fee, signing a three-year contract.
Boro beat off competition from two other Championship clubs to secure the highly-rated Doncaster captain, who was named Player of the Year and Player's Player of the Year at The Keepmoat Stadium in 2011-12.
Barnstaple-born George started his career at Exeter City and helped the West Country side regain their Football League status with a Conference play-off triumph over Cambridge United at Wembley in 2008.
George, who can play at either left-back or centre-back, made his debut in the 2-1 Capital One Cup win at Bury in August 2012.
A popular figure among fans, Friend was named in the PFA Championship Team of the Year for both the 2014-15 and 2015-16 seasons for his adventurous performances from left-back.
Following Boro's promotion to the Premier League in May 2016, Friend signed a new contract to keep him at the Riverside until 2020.