If their other children both have Celtic names (i.e. Irish or Scottish) why not continue that theme and choose another. Examples would be;
Aileen
Aisling
Brighid
Catriona
Finola or Fiona
Greer
Kiera
Kirstie
Maura or Maureen
Moira
Oona
I suggest searching Celtic names for girls on Behind the Name for a complete list of Irish, Scottish, and Welsh names. They are bound to find something they like that's different than the two trendy, tired, names they can't agree on.
Re: A poll for a friend...girl names