It sounds like you have listed your house with a real estate agent already. If that is the case, then yes, your friend must go through her. (Your agent invests time and money to list your house, and as part of your agreement, you have an obligation to refer any interested parties to him/her).
But if you have'nt signed a listing contract with an agent yet, and you have a buyer, then you don't need to go through an agent. Hoewever, it might be wise to ask one to help you manage the sale for a smaller fee than a full listing commission. Since the agent won't have to invest time and money in advertising, most would be willing to do this, and you could save money in the long run. An agent could review the offer and terms with you, work with the buye'rs lender, review the inspection results with you, and make sure you have all your paperwork in order, and coordinate the closing.