AMPscript loop function Question

Here's how I'd approach this -- loop twice through the subscriber's events with the first loop qualifying the events with a matching event_name and looping again to build the output string.

In the recommendation section, you can use the indexOf() function to determine if a person has attended a particular series of events or not and then output the content that's in line with your requirements.

%%[ 
    set @debug = 1


    /* pull email address from system string */
    set @EmailAddress = AttributeValue("emailaddr")
    set @firstName = AttributeValue("FirstName")
    set @attendedEvents = LookupRows('event','Email_address', @EmailAddress)
    set @rowCount = rowcount(@attendedEvents)

    if @debug == 1 then
      output(concat("<br>rowCount: ", @rowCount))
    endif

    set @attendedEventNames = ""
    set @attendedEventIds = ""
    set @qualifiedEventCount = 0
     
    /* qualified event loop -- events with a webinar names match */ 
    for @i = 1 TO @rowCount do

        set @row = row(@attendedEvents,@i) 
        set @eventID = field(@row,'Event_ID')
        set @event_name = Lookup("Webinar names", "event_name", "Event_ID", @eventID)

        if @debug == 1 then
          output(concat("<br>eventID: ", @eventID))
          output(concat("<br>event_name: ", @rowCount))
        endif

        if not empty(@event_name) then 

            set @qualifiedEventCount = add(@qualifiedEventCount,1)
            set @attendedEventNames = concat(@attendedEventNames, "|", @event_name)
            set @attendedEventIds =  concat(@attendedEventIds, "|", @eventId)

            /* strip leading delimiter from first element */
            if @qualifiedEventCount == 1 then
                set @attendedEventNames = replace(@attendedEventNames,"|","")
                set @attendedEventIds = replace(@attendedEventIds,"|","")
            endif

        endif

    next @i

    if @debug == 1 then
      output(concat("<br>qualifiedEventCount: ", @qualifiedEventCount))
      output(concat("<br>attendedEventNames: ", @attendedEventNames))
      output(concat("<br>attendedEventIds: ", @attendedEventIds))
    endif    

    if @qualifiedEventCount == 0 then
        raiseError("No qualifed events for subscriber", 1) /* skip the send to this subscriber */
    endif

    set @attendedEventNamesOutput = ""
    set @qualifiedEventNamesRowset = buildrowsetfromstring(@qualifiedEventNamesRowset,"|")
    set @rowcount = rowCount(@qualifiedEventNamesRowset)

    if @debug == 1 then
      output(concat("<br>rowCount (2): ", @rowCount))
    endif 

    /* qualified event output loop -- build a punctuated output string */ 
    for @i = 1 to @rowcount do

        set @row = row(@qualifiedEventNamesRowset, @i) 
        set @eventName = field(@row,1)

        if @i == 1 then
          set @attendedEventNamesOutput = @eventName
        elseif @i == @qualifiedEventCount then
          set @attendedEventNamesOutput = concat(@attendedEventNamesOutput, " and ", @eventName)
        else 
          set @attendedEventNamesOutput = concat(@attendedEventNamesOutput, ", ", @eventName)  
        endif 

    next @i

]%%

Hello, %%=properCase(@firstName)=%%.  Thanks for watching %%=v(@attendedEventNamesOutput)=%%.
<br><br>Here is more content you might like: 

%%[ /* output content based on events attended  */ ]%%

%%[ if indexOf(@attendedEventIds, "12345") > 0 and indexOf(@attendedEventIds, "23456") == 0 and indexOf(@attendedEventIds, "9876") == 0 then ]%%

    %%[ /* attended 12345, but not 23456 and 9876  */ ]%%
    <p>CONTENT HERE</p>

%%[ elseif indexOf(@attendedEventIds, "2345") == 0 and indexOf(@attendedEventIds, "789") == 0 then ]%%

    %%[ /* attended 2345, but not 789    */ ]%%
    <p>CONTENT HERE</p>

%%[ endif ]%%