I invited my dad's cousin and her husband, and they sent back the RSVP to indicate they were not coming. Now her son has gotten in contact with my cousin (no idea why he called my cousin instead of, you know, me or my parents) to say that he wants to come in their place? I can't use the "budget/venue restrictions will not allow me to accommodate extra guests" line because his parents RSVP'd no and he wants to go *instead* of them. What can I do?