Is an offer letter legally binding in Virginia?

My Finace recieved an offer letter from a new employer, She works in D.C. She signed and returned the offer letter and was planning on turnung in her notice to her current client the next day. When she went in to turn in her notice, before she could her current client offered her a tele commuting position that she just got approved by her uppers. She now is very concerned that the offer letter is a binding contract. I told her unless it is a "contract" it is not binding... She forwarded the letter to me and it does sound legal but is NOT a contract. It is an offer letter. I tried to research a little before consulting an attorney and found this site... Could someone please help ASAP. She has 24 hours to accept or decline the tele commute