"Our thoughts are with Lorraine as she recuperates following her unfortunate accident," Mrs Brown said.
"She’s such a good friend of PiggyBankKids and we would like to thank her for the contribution she has made and wish her a speedy recovery."
A spokesman for Chessington Equestrian Centre, where the accident took place, said: "Lorraine was having a private lesson and was supervised at all times."
Miss Kelly, a regular Twitter user, had used her account to keep fans updated with her training.
Discussing the challenge, Mrs Brown said: "We’re raising money for my charity PiggyBankKids, which works hard to give children the best possible chance of living a healthy and happy life here in the UK."