A recipient forwards an EXM email to a colleague. How is this handled?

If the initial recipient views the email then the viewing will be seen by Sitecore and then pushed to Salesforce when the scheduler runs. The email is seen by Sitecore because the email contents contain an invisible image defined like:

<img border=0 width=1 height=1 id="_x0000_i1027" src=http://officecore.fuseit.com/sitecore%20modules/shell/EmailCampaign... style='border-color:transparent'>

This identifies the initial recipient but if that recipient then forwards the email to a second recipient who opens the email, Sitecore will record the activity as from the initial recipient. From this, S4S EXM will also identify it as activity from the initial recipient.