Sweet viburnum is used in Native American medicine, however what I've read about it is:
"The fruit of some species (e.g. V. lentago) are edible and can be eaten either raw or for making jam, while other species (e.g. V. opulus) are mildly toxic and can cause vomiting if eaten in quantity."
I would say that having viburnum bushes in the habitat probably wouldn't be harmful to the tortoise unless he started eating too much of it. If there is other grasses and weeds for him to graze on, he'll probably leave it alone.