Although a sympathy note is difficult to write, the act of sending a card says as much as anything else. Use a blank card or one that just says, "My Condolences." This way, nothing interferes with your personal, handwritten, sincere expression of sympathy.
What to Write
Start by telling the recipient how you feel. Read the note aloud to make sure it sounds like you. Describe what the deceased meant to you and what characteristics or actions you remember. Amplify that with a personal story, even if it's simple. It is fine to write about a cheerful or funny memory. You might close by asking what you can do for your friend. If you and the remaining family members are religious, write, "My prayers are with you." If you are unsure about religiousness, err on the side of caution and stick with a secular card and sentiments.
What Not to Write
Do not offer advice, pep talks, opinions on the afterlife or comment on how long the parents lived. This likely will not be received in the spirit in which you intended. Do not say you know how your friend feels, even if you have lost a parent. The loss is not about you. Do not type the letter or send it via email. Do not wait until you can think of something better to say -- send it as soon as possible. A card with pre-written sentiments is appropriate if you do not know the bereaved very well, but you still should include a personal line such as "I'm so sorry for your loss." You might offer your help in any way, and if you are a neighbor, volunteer to pick up her mail if the funeral is in another town.