Did a girl make a mistake by leaving charitable work after she noticed mixed intentions from some participants in it, which made her lose the feeling of Allah's blessing in the work?

It would have been better to continue the work and advise the participants to correct their intentions and dedicate their work solely to Allah. You are not responsible for the corruption of their intentions as long as you have made your intention and work sincere for Allah. One should not abandon good deeds due to such pretexts, as this is from the whisperings of Satan to deter a person from good. So, cooperate with the sisters in righteousness and piety and in what pleases Allah.