If it's true, then it's not libel. A few pictures and the box the
plants came in should be adequate proof, but it's unlikely he needs
it, because no one is going to waster their time on nonsense like
this.

As for recommending people not do business with them, that's clearly
personal opinion, which he's entitled to give.

If he paid by credit card, he has some recourse. It's not uncommon to
have probems of this type with mail order nurseries.