Parkinson's disease often affects the autonomic nervous system ("internal nervous system") which is responsible for why patients with Parkinson's disease often have constipation, bladder urgency, and can have low and/or
swinging blood pressure.
In addition, the medications that we use to treat Parkinson's disease including carbidopa/levodopa (Sinemet) and the dopamine agonists (Mirapex and Requip) can lower patient's blood pressure. If there were no immediate changes in medication, we often look for dehydration and/or infections (urinary tract or pneumonia) that can throw things out of whack.
In the future, I would reccomend that your friend's husband take her blood pressure when standing up (not seated like normal) and if the top number (systolic) is routinely below 100, she will be at risk for fainting sometimes. Its often helpful to maintain good hydration, and we often encourage patients to liberalize their salt intake which helps maintain BP if someone usually runs low. She should check with her Primary Care before doing this however, as if patients have elevated BP, salt can make this worse.