Chocolate.
It makes sense what the vet says. Mud on the tennis ball becomes a grit/grinding paste on the teeth. He offered suggested alternatives.
What dog wants, dog usually gets. Not giving a tennis ball to a lab is like taking a doll away from a girl. Unusual and cruel.
Chocolate, tennis balls, frisbees, sticks, belly pats, etc.
Besides, the stuff they’re fed usually doesn’t require cutting with their teeth. Not like they’re going to need to tear into a mastodon or something.
I think most people are aware of not giving a dog chocolate. But grapes & raisins they may not be familiar with.