Very good evening doctors. my friend tested negative for covid on january 17, then tested positive on january 20, then just tested negative this past saturday, 2 days ago. would you know how that could be possible?

You did not say what type of tests were taken, but if they were rapid (antigen) tests, they may show a false negative if the viral loads are low, such as in someone without symptoms. ref: https://health.osu.edu/health/virus-and-infection/home-covid-tests

The test that was positive should be considered a positive test. There may be false positives and negatives associated with screening. The antigen test is quite accurate if positive. Watch for signs of shortness of breath, fever and cough. Continue isolation until at least day 5 without symptoms.