So, I just did something similar to this today. Firstly, only she can use the e-credit so it will have to come from her account. You should be able to gift your miles to her and they will appear in her account (I had my wife do this so I could get my miles total up to cover all of her/kids tickets and then used e-credit for mine). Just know not all routes/flights will be eligible for miles+cash/e-credit - I had to do some schedule adjustments to work around this.