Joint student tenancy

Hi my daughter signed a joint tenancy with 4 other students, paid a £450 holding fee each and £250 deposit and 1 months rent each the tenancy was signed but she decided not to go to university. She found another tenant to take her room who has paid 6 months advance rent and a deposit. The agents have only got 2 of the 5 girls to sign the new tenancy but are withholding my daughter’s paid rent and deposit til they all sign. Can they do this?