FIRST: Don't make excuses or apologies for this because it really wasn't any of your doing. You're in the same boat (for the most part) as the other staffers. She didn't approach you and you didn't suggest the exception.
I assume that the three people in that position are doing about the same amount of work, but she's probably more efficient at the job. It will probably cost the friendship, but that was dead when she got preferential treatment anyway, however it came about. As you say, it's really out of your hands. Others higher than you made this decision and unless the higher up says different on Monday, it's a done deal.
Instead of expecting the other two people to keep up their same outputs with fewer hours, you have to redistribute the workload fairly. It's the only way you, as a supervisor, can make the situation more palatable for the staff.
If she's got more time, she should be doing more work than the others, especially if she's The Best.
You need to propose it nicely on Monday to the higher up if s/he approves the full hours exception.
Make it clear that this is a fair way to handle the exception that will minimize the other staff's disgruntlement.
When it's all approved, tell her the deal, with someone else in the room, before the rest of the staff. If/when she voices an objection, tell her that, as a supervisor, you have to be fair and equitable towards your ENTIRE staff.
After she's been told, you should let the rest of the staff know that the hours exception was out of your hands. If you want to be nice, you could show the higher workload as something she agreed to for the good of the office. Emphasize that you're making the best of an uncomfortable situation.
If she runs out of work, make some for her. Do NOT let her sit idle and get paid for it - it's horrible for morale and will build resentment.
You will need to come up with a plan (or "spin") for this specific problem: while she's taking her vacation time in the summer, the other two people will be picking up her workload. So, while she's going to be doing more now, they'll be doing more over the summer, probably with still-reduced hours. Is the workload lighter in the summer, I hope? That might smooth it over.
If there is an overtime possibility, the other two people should get first refusal rights. Don't say that out loud, but keep it in your back pocket. You can justify it by their reduced hours - it's just straight time for them, but keeping her longer can result in time-and-a-half. That's just business and management.
Good luck, it's really difficult to manage and motivate in this horrible economy. I hope your business picks up and everyone gets their hours back very soon.