Every supplier should have their own contract with their own T&Cs, but generally I think you'll find that deposits are non-refundable unless it's the supplier themselves who cannot fulfil their contract.
Any deposits or payments made to (and retained by) the supplier do need to be reasonable however. "Reasonable" isn't a set sum or %. Nor does it mean what you or they think they should be able to keep.
Reasonable relates to actual costs they have incurred until this point. This may be for things like stock, supplies, business rates, administration and of course their time investment until that point. Depending on the service, a deposit can usually be anything up to around 50% of the total cost.
A supplier has every right to withhold a reasonable sum to cover their costs until this point and that is usually what a deposit is for- to ensure their costs are covered and they are not at a loss in the event a couple need to cancel for any reason.
Unless the wedding is in a lockdown period or their particular profession is not permitted to providing their service at the moment (ie. no singing is allowed at weddings at the moment, so presumably wedding singers can't perform) then it is sadly your daughters choice to cancel- even if it's not necessarily what her first choice would be.
Hopefully she has wedding insurance that are willing to cover lost deposits?