Neil
US co-pilots don't need type ratings under FAA rules - the problem is that the FAA rules do not comply with the requirements of Annex 1 to the Chicago Convention on this point
While this is fine in the US, problems can arise elsewhere as other states are not obliged to recognise licences which do which do not meet the requirements of Annex1. The UK, for example, requires US co-pilots to be compliant with the type rating requirements of Annex 1 if they wish to operate into the UK.