Misscarriages usually happen for a reason, either a week fetus or placenta or ill health of the mother. Other times it can be due to improper hormone balance. Misscarriages happen for various reasons, and no one ever really knows why.
To me though having 4 in one years seems kind of suspect. I would suggest she get checked from her midwife or perhaps see an OB/GYN. Not knowing the whole situation I'm just guessing here, but going by her age it could be a few things from: improper hormone balance to uterine fibroids. That is why it is best to get checked out thoroughly.
There is no need to worry about tissue being left behind after the misscarriage your friend would know if that was happening because of the pain and symptoms involved.