If she is depressed and her grades slipped, then you can't punish her for that. One cannot control their mental health without help from either a professional or possibly even medication.
Another question, how far did her grades slip that she lost aid? My GPA dropped to a 3.45 and I was put on probation for my scholarship - I had to maintain a 3.5. I was a mess and my grades suffered. I felt as if nothing was in my control anymore and had no idea how to cope. Yes the probation served as a kick in the pants and I got it together, but it was a huge struggle. If your DD is feeling like this, please don't punish her for the grades. She may be doing the best she can right now.
I just saw the post about the depression.
OP- you have more to consider that the expense of a trip. Your DD has had a rough year, is clearly not adjusting to being away from home, has lost her financial aid, and your concern is "fairness to the younger so she can bring a friend?" Now it is no secret I woudl never leave one of my kids home because 18 is no magic number in my family. Shoot, when my nephews were in teh military they got Christmas gifts, no matter what age. You serve, I remember. I think that you need to take a closer look into what is bothering your daughter, and address that. I get that you want to honor some commitment you made to the youngest, or make things equal, whatever your reasoning is, but do you not worry that you are placing hurt feelings over teh very real concern that leaving your oldest behind may cause more damage than you can repair later?
One thing I always did and still do with my kids, and my "baby" is 38 BTW, is tell them that nothing is ever "even" in the G household. I do what I feel is the best thing given the circumstances. That means if one of my kids cannot join us due to unforeseen financial circumstances we kick it in. We do not share this with the others, it is just done. If I see something for one, I get it, I'll see something for one of the others another time. DH and I set this expectation when thye were young, and this has always served us all well. My DDIL, before she and my DS were married commented that I had 2 sons when I had done something extra for my DD> I replied "Yes I do, and right now my DD need a little more. YOu just pray to GOd you are not the one that is in a position you and DS need us. That is not where anyoen wants to be. But if it happens, we will be there and not one of the others will even blink. SHe never said another thing but one day she and I were chatting and she said she had learned more about fairness in our family because for us, fair was not even...it was what was given freely at the time when it was needed the most.
Be fair to your older DD, she needs you now.