There's really not any information on the subject. This is on of the only article in English I've found which seems to contain factual information. And there is this testimony of someone who went on a truffle farm.
From other articles I've read (mainly in French), here's what I gathered :
Although there are some breeds more suitable for this, and it is best to train a puppy, any dog can be theoretically trained to be a truffle dog, even a rescue dog.
The training for a truffle dog can be very gentle. There is a "university" to train truffle dogs in Italy. Their method to train is to make this a game for the dog : find the truffle, and you get a treat. Same for this woman who trains truffle dogs in England.
The issue is that even if it is possible to have a truffle dog and treat it well, there are still people who will abuse an animal if they only see it as a tool. Here's a french article where a truffle hunter puts a weird muzzle on the dog to be sure he doesn't eat the truffles. The article also states the dogs were kept in small cages, only being let out to hunt truffles. I've also read in several articles that people poison the dogs of their rivals.
Last note : I tried the Peta website to see what they have to say, instead they have several recipes with truffle in it, so I guess they consider them vegan...