Passenger Side Seatbelt Smells Putrid

I regularly drive my good friend and me to the gym. Recently, I’ve noticed that the passenger seatbelt smells really bad. Like… when you sit in the passenger seat, the smell is unavoidable and hits you immediately.

I’m pretty sure it’s coming from my friend, but I genuinely don’t have the heart to bring it up and hurt her feelings. Are there any preventative measures or ways to fix this without having an awkward conversation?