No, she can't have 20 ranks in it, because she's at MOST level 5.
Level 5 means at most 5 ranks in acrobatics.
If she's pulling off this, then she's likely to have 19 DEX or so, (Max human starting dex is 18, +1 at level 4), and acrobatics as a class skill giving her + 12 to her roll in total, before shit like Skill Focus for + 3, and Acrobatic for + 2, for a total of +17.
So her acrobatics roll is d20+17.
IF we're doing pathfinder, and she's level 5.