ANSWER:

If this extra money was never hinted or talked about when taking the loan and she just gave it as a gift then no problem in accepting it. 

I had given my friend some money as a loan. When she returned it back, she gave me some money extra ( as a gift expressing gratitude for helping her in times of need). I never, on my own accord, as her to give me extra money. Does this count as taking Interest (Riba)?

Loans, Riba | October 11, 2018 5:46 am
Previous

is the agreement islamically valid if I pay a heavy deposit and don’t pay the monthly rent with mutual understanding and take possession of a house?
Next

Sheikh, if i m asking my brother for some money (with the mindset of returning him back) , so, it means I will fall in the category of those people who doesn’t put their trust in Allah (SWT)?