I think Stargazer said it best: triage is an extremely subjective area. One man's emergency is another man's, well, non-emergency. There's a lot of variables we don't know.. the experience of both nurses, the other patients in the waiting room, what was going on in the ER at the time, your father's presentation, and so on. Don't take it as the ER not taking your father's complaint seriously. Fifteen minutes is not a long wait at all and at a high level ER, this would actually be considered a short wait time. Sometimes when you're the patient or with a patient, it's hard to step back and look at the big picture of the ER. Yes, you might be worried and yes, it might be an urgent problem that should be addressed by the ER, but as always, emergent complaints take priority.
Hope your Dad is ok though.