The Bow Limit is described on page 102 of the big rules book. Most Armies ( Factions ect ) have a 1/3 Bow Limit meaning a force of 30 models can have 10 with bows. Heros are not included in Bow Limit. If your Army contains more than one Faction then the Bow Limit is done individually for each ally. So a Isengard and Mordor Army made up of 30 Mordor models and 30 Isengard models can have 10 Mordor and 10 Isengard models with bows.
Some Factions however do not have a normal bow limit. In your case the Rangers of Ithilien are part of the Minas Tirith Army list which has the normal 1/3 Bow Limit. So your friends list was not legal however now you know for next time. Factions I know of with different Bow Limits are Warriors and Riders of Rohan ( those models don't count towards a bow limit but everything else does ), Grey Company ( Complicated! See Sourcebook for details ) Harad and Umbar have a 1/2 Bow Limit and so does the Eastern Kingdoms and Thranduils Hall has a 100% Bow Limit. Hope this helps!
