Yeah, that would be copywright infringement. That is a known song, and you’d have to get the rights to the music. Just like the company that made the ring tone had to pay for the right to make it a ring tone.
But like Sean said. If they are insisting, have them sign a waiver so that all responsibility is theirs when it rears it’s ugly head.
The issue, not your client. The issue rears it’s ugly….oh nevermind.