There is a risk with tennis balls. The lining used on them are generally a type of rubber, which can very easily be chewed off and ingested, not to mention if they get to crack open the tennis ball, which is made from rubber. Both of these events can lead to a life-threatening blockage. Which is why we say to keep rubber and foam items well away from ferrets.
You're better off with some hard plastic jingle balls (don't get ones with pieces that are easily chewed off, like little spikes). But because there is no toy capable of withstanding a determined chewing ferret, it's just good practice to do regular toy checks to make sure there is nothing damaged.
Definitely nothing containing rubber or foam. As Lindale mentioned it is a major blockage hazard.
Ferrets: Contessa, Frankenfurter, Butterscotch. Kitties: Watson, Oskar DIP Sinnead, Vincent, Boris, Zeus and Athena, Willow, Mr. Frodo, Indie, Lucrezia, Judge, Odin, Miss Emily, Suki, Cody, Aristotle. RIP Herne, Ligeia, and Mr. Stubbs
No, I have a couple that I use to interact play but they're not allowed to have them. I do have a couple of real tennis balls, which they're allowed to play with but I watch the flocking very closely and as soon as it starts to wear, the tennis ball is tossed (blockage hazard) ciao