GreenGardenGnome I totally agree.
cat sadly no idea, but there may be safe chew items (look at sites providing fiddle toys for autistic children, my dd is probably on the spectrum and we found chewlery which might help).
I feel quite strongly about this because dd is a teenager and still very occasionally sucks her thumb! we tried everything! I think one thing that does work is that eventually they care what others think and they self modify. But I would never encourage friends or teachers to make it a shame issue. any reminders should be so subtle other children do not pick up on it. E.G. a tap on the child's desk, and should be agreed with child in advance. Other things could work at home but again they should be agree, especially if involving something like a light tap on child's shoulder. I'd say that would be too much from a teacher and better for a parent to do.
www.arktherapeutic.com/blog/thumb-finger-sucking-alternatives/
This is so important... "Thumbsucking lasting beyond age 5 can usually be prevented if you avoid pulling your child's thumb out of his mouth at any age. Also, don't comment in your child's presence about your dissatisfaction with the habit. Scolding, slapping the hand, or other punishments will only make your child dig in his heels about thumbsucking. If you can wait, your child will usually give up the thumbsucking naturally. If you turn the issue into a showdown, you will lose, since the thumb belongs to your child."
community.babycenter.com/post/a21706963/need_soothing_alternative_to_thumb_sucking
I looked at photos of protruding teeth with my dd and her teeth do protrude a bit. She will need a brace but may have needed one anyway.
Try all the usual things but do not induce shame or make the child feel babyish for doing it. Insecurity will most likely only lead to.... more thumb sucking!
OP it is not your responsibility to help this family but I found out loads on the internet, I'd gently push them to push themselves in the right direction and I'd empathize that the child wanting to stop may be key.