It's very, very rare for a dog to start biting truly 'out of the blue'. Much more often, we miss the signs that there's escalating problems, or even accidentally put the dog in a situation where they feel they have no other option but to bite. That being said, he should have a thorough veterinary workup if he didn't get one when this behavior change happened, in case there is a medical problem underlying the biting. A medical condition that impacts a dog's mood, behavior, or defensiveness due to pain is the only thing I know of that can cause genuinely sudden severe biting with no leadup or forewarning.
But as I said. We miss a lot if we don't know what to look for. Dogs give us tons of body language when they're uncomfortable - lip licking, side-eye, tight lips, etc. - that humans aren't hard-wired to recognize. There's lots of incidents of dogs who have been giving these signals for a long time to try to tell their owners that they're uncomfortable in a situation, but when they finally do resort to biting because everything else has been ignored, the owner assumes it's 'out of the blue' when really they've just missed important communication.
Similarly, some people think that they're reducing or preventing aggression by correcting more obvious ways dogs tell us they're uncomfortable - like growling or air snapping. However, this only means the dog stops growling and air snapping, not that the dog becomes more comfortable/less fearful in the situations it used to growl in. It's a real risk that these dogs will eventually be pushed past their limits and jump immediately from seeming 'calm' to a full-on bite because they've had all their obvious 'leave me alone' signals suppressed.
Your friend is in a dangerous situation. A dog that's given someone stitches needs IMMEDIATE professional intervention. In the meantime, she needs to be employing a muzzle and keeping the dog away from contact with other people. That might not be permanent, but if she doesn't know the dog's triggers, it's her responsibility to protect the people around her from her dog.
Even if your friend were here giving us way more information about the bite and history of this dog, I'd say the same thing. We can't help. Rule out medical causes as thoroughly as possible. Then she needs a behaviorist - preferably someone certified through a reputable third part organization like APDT, IAABC, or CCPDT (all orgs have search functions on their websites to find trainers/behaviorists near you) - who has experience with bite cases. She needs to work with this person to figure out why this is happening, what she's missing, and what's triggering her dog, then figure out if there is a behavioral plan that can work for them. It'll probably be expensive. So will court fees be if her dog injures the wrong person (or, goodness forbid, a child).
If she's not willing to do this - and especially if she's not willing to implement restrictions to keep other people safe from bites - she needs to consider euthanizing the dog. I don't say this lightly. But this dog needs intervention. Rehoming isn't even an option, because rehoming a dog that's caused severe damage with their bites (and stitches are pretty darn worrying) is extremely irresponsible and can (again) open your friend up to legal action if the dog injures someone in their new home.