"We tried to entertain the idea and it felt so scary and overwhelming for the medical staff and her mother that finally I said no, and my heart broke again."
Kramer said Chen burst into tears of joy at the sight of the famous singer.
Chen shared video footage of the concert surprise on Instagram on Friday, when she heaped praise on Welch for making her girl's wish come true.
"@florence of #Florence&TheMachine is such a beautiful and kind soul!!" the proud mum captioned the clip.
"She brought so much joy to all of us today. What a Blessing!!"
Among the tunes Welch sang for the teen included Shake It Out and Dog Days Are Over, for which the star was accompanied by guitarist Rob Ackroyd, as a group of the girl's friends looked on in excitement.
"There were shrieks and squeals and so much joyous music and singing and tears and tears and tears of grief and joy and hope and love and utter devastation," Kramer said.
"And somehow simple kindness made everything better," she added.
- WENN