Martin (1992: 131-2):
Finally the problem of realising more than one participant in a nominal group needs to be addressed. Clear cases involved nominal groups embedded in the Deictic and Qualifier functions and can be passed over quickly here; for example three participants, John, Mary and the friend, are realised in a group like John's friend who Mary had never met.
This continues the confusion between clause participant and nominal group Thing. To be clear, only ranking nominal groups realise (direct) participants. Embedded nominal groups, such as those realising the Deictics or Qualifiers of nominal groups, do not realise participants.
To be clear, in SFL theory, the nominal group John's friend who Mary had never met potentially realises one participant (in a process), but it doesn't realise a participant unless it is construed as such in a ranking clause.