The following article found in the ARC home page offers some advice.
Sexual abuse includes a wide range of sexual activities that are forced upon someone.
People with mental retardation are often unable to choose to stop abuse due to a lack of understanding of what is happening during abuse, the extreme pressure to acquiesce out of fear, a need of acceptance from the abuser or having a dependent relationship with the abuser. The rate of sexual victimization in the general population is alarming, yet largely goes unnoticed. Although these figures are disturbingly high, people with mental retardation and other developmental disabilities are at an even greater risk of sexual victimization. More than 90 percent of people with developmental disabilities will experience sexual abuse at some point in their lives. Other studies suggest that 39 to 68 percent of girls and 16 to 30 percent of boys will be sexually abused before their eighteenth birthday.