Here’s what the US Gov says about it:
https://www.howto.gov/social-media/video/learn-about-video-copyright
You can never assume someone is not going to go after the client, or you, when you misuse copyrighted materials. Here’s an example which happened years ago, when I was creative director at a small cable station in NH. We had a show that need a theme song, and there was a very good acoustic guitarist who had been on one of our shows. We contacted him, and signed a contract agreement with him that we could use the music for a nominal charge (maybe 100 dollars). Once the show got on the air (bear in mind that this show was in a minor market (less than 20,000 viewers, on a good day) he decided that he should have been paid more, and sued us for 100 thousand dollars. We won the lawsuit, but it took time, and cost thousands of dollars in lawyers fees – and this was with a signed release!
Personally, I would walk away from the client if they could not see the serious mistake they’d be making. You might want to send them a couple of links to show how seriously this is viewed, and how actively the copyright holders are going after people:
https://abcnews.go.com/Technology/wedding-photographers-fear-sued-music-publishers-online-videos/story?id=15138195
That link is about wedding photographers being chased down. One would think it might be a pretty low-profile thing, but once it hits YouTube, it will become a target.
Joe Bourke
Owner/Creative Director
Bourke Media
http://www.bourkemedia.com