The weapons section such starts on p88 and has rules for all the generic weapons in the game, along with any special rules they have. Weapons such as halberds and great weapons have the 'requires two hands' special rule, which is found on p75 (in the special rules section). This clearly explains exactly how and when models with such weapons and shields can and cannot use their shield. Point your opponent to this next time, s/he is most definitely wrong.