Should the fruit_ref field's value come from the SharePoint list's fruit_ref column? I think filtering should work just fine. So set a default value for your fruit_ref field, that is - the value of the SharePoint list data source's fruit reference filtered such that fruit = your main data source's fruit (combo box) field.
It may look something like:
ShP_Fruit_Ref[ShP_Fruit = Form_Fruit]
or using XPath:
xdXDocument:GetDOM("Fruits")/dfs:myFields/dfs:dataFields/d:SharePointListItem_RW/d:ShP_Fruit_Ref[../d:ShP_Fruit = xdXDocument:get-DOM()/my:myFields/my:Form_Fruit]
Does that make sense?