How do you prove you didn't video someone (changing) while they are slandering you otherwise? Life altering accusation.

My son's female, roommate is showing everyone a video of my son's phone sitting in her window from outside, claiming he was videoing her. She's already contacted his work, which happens to also be where he lived and demanded they make him move immediately and file a police report. He is devastated. He was on the patio smoking and had his phone plugged in. Going to go to geek squad to have them verify there is no video. Any better advice? Thanks, Angel