Karela, your husband's chances of having the gene are considered to be 25 percent, or half of his mother's 50 percent chance.
Although age of onset can range from 6 months into the 80s, most.people with the gene will become symptomatic in their middle years. The average age of onset is 39.5. Every year after that that your MIL lived and didn't have HD symptoms, the likelihood that she had the gene decreased.
Presymptomatic testing is a highly individual decision. Many people prefer not to know but some find living with uncertainty very difficult. Sometimes people who have decided not to test will change their minds as they get older and find that HD is occupying their thoughts too much and they are constantly looking for symptoms in themselves. There is no right or wrong decision, just what is best for the person at the time.
I will be thinking about you and Rob, hoping he is negative for the gene, and wishing you strength!