I couldn’t find very much explicit advice about what a mediator could do to support someone who was observing another’s emotional expression. For example, when a party is crying what should the mediator do to support the person who is not crying; or when a party is very angry, what should the mediator do to support the person who is the target of that anger.
Generally, the advice seemed to be move to separate sessions and try to spend equal time with each party, irrespective of whether they were the person expressing strong emotion or the person observing it / to whom it was directed.