If a contract isn't signed by both parties, is it legally binding?

A friend of mine signed a contract but the person who wrote the contract did not sign the document. Is she legally bound to fulfill the terms of the contract? She wants to break it and wonders if it is a legally binding document.

A signature is not, generally speaking, a necessary element of a contract. It is only evidence of a contract.

Unless local law requires that a contract concerning the subject matter involved in this contract be in writing and signed by the parties (which usually is the case when real estate is involved) then the failure of one party to sign does not void the contract.

it is also possilbe for a contract to contain a clause which makes singnatures by both parties a condition for its validity but without the contract in front of me I can't evalute specific language that may or may not be there.

If the person has the copy of the contract your friend signed, they consider it to be signed and can produce a signed copy if your friend wants to break it.

There is another view that since both partioes have been fulfilling the obligations of the contract it constitutes a verbal agreement (if not signed) and is therefore legal and binding.

That depends - if it is for real estate - it is not binding.

No it is not