Agree with @blossom at #94. Your D is doing right working all sides, trying to get an extension from one and accelerating the other. You might even get some intelligence on the respective firms based on their reactions.
How many live “choices” are left that she would rather take than the existing offer? Is it just the 2? If the second tier is more of a regional house vs a tier 1A, I might be tempted to hold out for a say GS or MS. If you like, you can PM me with the exact choices and departments.
Once having accepted an offer though, I think you need to stick with it. It’s a pretty small world, and while the chances of anyone remembering letting alone holding onto a grudge against a summer intern are small, you could hit the one person who has a long memory and who will hold a grudge.