15ppm of Nitrates is not a problems. I have kept numerous inverts for long time with much higher nitrate than that.
If your friend had invert tank, Cu is not the reason, at least in your friend'd tank. It wont hurt to check both tank and tap when possible.
If specific gravity was swinging due to evaporation while away for long time, such can also cause this.
Strong currents in the tank? Another possibility.
Unfortunately, Once LR and Substrate are exposed to heavy metals, it is close to impossible to free them of heavy metal and cost ineffective to be used for inverts.
high temp due to defective heater??
Water alone can be filtered with high quality carbons and heavy metal removing resins to remove Cu.
Nitrifying bacteria are the one which would yield Nitrate and Denitrifying bacteria are the one which will reduce nitrate.
Depending on LR, they can perform both.