My friend has eight children and she is pregnant. She is suffering from Dawali in her legs and she found it hard to serve more kids. She was asking me if she can have an operation to stop becoming pregnant and because I cannot have children yet. So I told her that kids are wonderful and Allah (SWT) will reward her for having educated Muslim kids. Later I felt guilty because she may take my word for it and I will be responsible about any harm that happens to her. So would you tell me by evidence from Qur'an or Sunnah can she have this operation? You should know that she tried every thing not to get pregnant and she could not. The doctors said this is the only way.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

You are not a sinner rather you are a well-doer, al-Hamdu Lillah. In fact, you will get good reward on your advice and guidance, Insha Allah.
As for family planning we have issued Fatwa concerning this matter, please read the Fatwa: 82154 and 84086.
Allah knows best.