A Great Dane is too large to do Schutzhund, even if the dog did have the right temperament. That is too much weight to be able to control a dog when it strikes. A dog is about 3 times its weight in comparison with the strength of a man. A Great Dane is just too big.
But the dog can provide protection in a safer, better way. Read the article at the following link--it's free, no registration or anything required. The site is provided as a public service by the Veterinary Information Network.
http://www.veterinarypartner.com/Content.plx?P=A&A=2019&S=1&SourceID=47
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Kathy Diamond Davis, author, "Therapy Dogs: Training Your Dog to Reach Others," 2nd edition, and the free Canine Behavior Series articles at http://www.veterinarypartner.com/Content.plx?P=SRC&S=1&SourceID=47