Given GPIO frequency limits, reproducing a beautiful sine wave for a 1000 kHz carrier is a real challenge. He should borrow an oscilloscope and measure the output waveform.
An oscilloscope is the wrong tool for that. You can tell the difference between kind-of-square and kind-of-sine wave using one, but hardly more. At least DSO often have a FFT option and on the new ones with 12bit ADC that might actually be more than just a gimmick.
You'd want to use a spectrum analyzer to verify that other frequencies are present only at very low levels. TinySA might be the cheapest option or an used Digilent Analog Discovery.
You'd want to use a spectrum analyzer to verify that other frequencies are present only at very low levels. TinySA might be the cheapest option or an used Digilent Analog Discovery.