There are dozens of things that could be causing poor grades in a first semester freshman, is there actually any tangible reason to suspect that video games are the cause?
Additionally, are you sure that those exam grades are actually poor grades in the first place? I know a 70-something percent can be shocking to a straight-A student in high school, but college is not high school, especially not for a physics major. It is not uncommon for exam averages to be in the 50 - 70% range, especially in STEM weed-out courses. It’s a hard pill to swallow for many people, but it’s just the reality of many rigorous programs.