Hi and welcome to the WAB forum.
The bird really needs to be kept in care until it next moults into new plumage, but its current state may be due to the conditions it has been kept in, and the food it has been supplied.
To keep their feathers in good condition, birds need to be able to bathe (or at least shower) on a very regular basis ... cages need to be big enough for them to flap their wings without snagging on bars, etc. and food needs to contain an adequate supply of nutrients.
As it may be some months from gaining new plumage to replace that which has become damaged or soiled, then it would probably be best if you contact a wildlife rescue centre in your area, who might be prepared to help you (and the bird).
Let us know which part of the country you are in and we can supply some possible contact details ... or there is a list of some rescue groups on this website at :
http://www.wildaboutbritain.co.uk/maps/rescue-centres (zoom in on the map for your region).