As others have already stated there are a variety of reasons why women sometimes have heavy periods - lasting from 7 to 10 days.
Though you haven't said your SIL's age - if she is in her late 30s to her 40s up to the early 50s it's not unusual that she could be experiencing the signs of menopause. This may or may not include the possiblity of her having fibroids which may be why there is a heavier than "normal" flow. That said the culprit behind them could be an imbalance in her hormone levels.
I would strongly suggest she put off the surgery & have her hormone levels evaluated prior to doing surgery as well as having a routine exam done to determine if there could be a cyst, fibroid or other growths going on. If fibroids are found - they can often be treated or surgically removed - though again the hormones need to looked at - as it may be her estrogen level is high & could a factor here. Estrogen levels do vary, and can be high in part due to diet, & for other reasons as well.
It used to be that hysterectomies where routinely proscribed by doctors as a cure all for almost anything it seemed - once a woman was done having children - as the male dominated profession saw little use for keeping an
organ that had served it's purpose!
They are still performed way more often than necessary & can cause a multitude of health concerns, either because of surgical complications, or because your body has just undergone major surgery & now has to deal with instant menopause. None of those possiblities are what I'd call a picnic & your SIL may be setting herself up for further problems to deal with in the future.
Traditional Chinese Medicine & accupuncture would very likely resolve her problems - as can other more complementary & alternative methods, but it may not be something that she is open to. There is also what is known as
the Arvigo Mayan Uterine Massage (all done externally). You can find info online about it - as it's fascinating ancient healing work with a highly successful cure rate for many reproductive area problems for men & women alike.
It may be that your SIL just wants a quick, & what she thinks is a simple end to an annoying problem. If she chooses to go through with the surgery there will likely be anger & depression, when she realizes that while one issue is dealt with - now more things she never even considered are cropping up.
You can only do so much to try to shift someone else's thinking - so keep that in mind when you discuss this decision with her.
My own SIL went through the same journey a few months ago - so I speaking from experience & knowing her I purposely chose not to get very involved as it is her body & her life! She was convinced it was her best option. (big sigh)
At the very least - read Dr. Christianne Northrup's books on Women's Bodies, Women's Wisdom & The Wisdom of Menopause & pass on the books to your SIL!