Huntington's disease is inherited autosomal dominantly. Since it dominant gene, you only need one of the alleles to make it expressed. Heritability of Huntington's disease is ranged from 50%-100%
If one of the parents is heterozygous (one defect gene, one normal) then 50% of the child will get it. If both parents are heterozygous (both defective gene) then 75% of the child will get the disease.
If the parent is homozygous (both defective gene) then 100% of the child will get the disease.
In this case, you must mean that the parent is homozygous. The ratio became 100% because the parent certainly give defective gene, and one defective gene is enough for the disease.