re: "No one in her family had cataract before."
As they can tell, everyone will eventually get cataracts if they live long enough. It isn't merely that old age can cause cataracts, it appears it always eventually does, so that merely means no one in her family has yet lived long enough to do so. The odds though are that the cause of a cataract at a young age is different than typical age related cataract.
I had a problem cataract appear at the atypically young age of 49 and haven't got a clue the reason, unfortunately in most cases people never know what caused a particular cataract. Cataracts at a young age are rarer, but as noted above there are many potential causes for them. There are some genetic disorders like galactosemia that can lead to cataracts, one weaker variant of it leaves people with some ability to digest milk so they may not realize they have it, but with a risk of cataracts from doing so.
Anomalychick is right on. Far and away the most common cause of cataracts is aging and the average age for cataract surgery is in the upper 70's. As she said there are many causes. These are what I would rank as the most important from my practice: heavy use of alcohol and tobacco, obesity and diabetes, heavy sun exposure, use of steroids (pills, inhalers, eyedrops) over extended period, family history, trauma to eye, systemic diseases that have cataracts as a component.
There are even congenital cataracts that children are born with. One of our neighbors brought in her 9 year old thinking she needed glasses because she wasn't seeing well. She had bilateral cataracts. She was worked up for underlying disease and none found. She had cataract surgery on both eyes at age 10.
If one can be grateful for a disease it would have to be that cataract surgery , for most people (not an internet eye forum for people with problems) do extremely well and good to great vision is restored.
I developed bilateral cataracts at age 25 for an unknown reason. There isn't necessarily a reason. Some contributing factors are sun exposure, tanning, smoking, drinking, radiation exposure, diabetes, certain metabolic disorders. Save for diabetes or metabolic disorders, or nuclear fallout, you can't really count the others as a contributing reason until age 45 or so, since you haven't said how young your friend is. I had no risk factors or family history.