Firstly, it is very, very kind of your friend and her husband to even consider doing such a wonderful thing for you. Very generous indeed. That said though, I would be very thorough and very cautious in making the decision to use her. She has a very good track record of fertility, but at 38, her eggs may not be what they once were. A lot happens between 35-38 with our eggs. If this is your "1" shot, you may want to consider going with a younger donor. If you are prepared for multiple tries, then that may not be a factor for you.
Your friendship is still relatively new. You may remain friends forever, but what if you do not. How will you both feel then? Will they "regret" being your donor if your friendship fails, one of you decides to move to another city, they question how you parent in a situation once the baby is born, etc? Are you ready for the emotional toll that it may have once the baby is born? Do you want your child to "know" that she is the egg donor? How will her children and your children interact....as mere friends or in school are they going to refer to one another as ...."that kid is my 1/2 sister/brother"? You all may agree to keep the news just between the 4 of you, but how are you going to feel if & when someone accidentally mentions something in your group of friends.
I am not suggesting that you should or should not use your friend as your donor. That is a hugely personal decision. There are a lot of future scenarios to consider though well beyond just getting pregnant so be sure you all are on the same page well in advance and have a legal, binding agreement that spells out everything. It's much easier at this point for your friend to think she can "detach" from her eggs, but she may feel differently when she sees your child playing along with hers on the playground and notes their similarities.
Whatever you decide, good luck to you and I hope everything works out for you.