Phone sends imsgs to SMS contacts

My friend switched from an iPhone to Samsung. My phone continues to send her msgs as imsg so she doesn't get my texts. Friend keeps telling my to turn off my iMessage function, but why should I give up I messaging all my contacts due to this one contact having an issue? I have changed contacts phone label from iPhone to mobile, and have also tried deleting this contact completely. My phone continues to iMessage the number when I text. What can I do besides turning off all iMessages?