Here is what I think

Your friend needs to tell the company which provides her with the first offer that she has been able to find a better job and therefore she wants to void her acceptance to the offer. After both your friend and the company agree to rescend the job offer, she can accept the second job offer. If they want to match the second offer, that's fine and she can ask for more money from the first company. In the end, it's her call. Hope this is helpful