"rep {faction name} +/- 0.9" where {faction name} is the name of the faction. Not sure if you have to use the short name (Can be found in FLStat. Should look something like "ga_grp" or something like that) or the long name. No need for "/" before rep and use ether + or - depending on whether or not you want it hostile or friendly.
In open singleplayer, type the following into the chat: rep fc_wild 1
The general template is rep <internal faction nickname> <number>
The number must be between -1 (full red) and 1 (full green).
You can find the internal faction nicknames in your freelancer folder at Freelancer\IONCROSS\gamedata_factions.txt