The only way to remove nitrates is with water changes (plants eat some too) so that vicious circle is what we do as fish keepers everyday. Fish make waste, chemical levels spike, we change water.
Let me clarify by saying I may not be as experienced as you. Was just something that surprised me. I spend most of my time on AC and in any situation with a chem spike water changes were the prescription, large percentage water changes at that.
In this situation he can always dose with prime but I would take mostly harmless nitrate over ammonia any day of the week. Ammonia will kill, nitrite will suffocate, nitrate won't do much of anything under 40ppm