By the time Hermione began practicing magic with Jacob, Harry had already headed back.
Harry did not quite understand what Hermione was thinking, but presumably she just wanted to improve her spellcasting skills? After all, comparatively speaking, Hermione really was somewhat lacking in that area.
Her grasp of theory was naturally first-rate. Even Harry, as he was now, might not understand some newer-generation magic as well as Hermione did.
But when it came to practical ability, never mind Harry-even Ron was now a little better than Hermione. Because of this, Hermione often studied in private. Her research was clearly headed in a somewhat misguided direction, though.
She tried to work out, through theory, why her magic failed to produce the expected effects when she used it, and naturally, this did not yield the results she hoped for.
Harry had heard Parvati muttering about it. Hermione was studying the use of magic even in their dormitory.
This left her with even less time than she already had under her heavy course load. Harry had originally wanted to find an opportunity to talk to Hermione.
For something like practical magic, reading books was no use.
She would have been better off spending that time asking Professor Lupin for advice.
Although the other professors were all highly accomplished wizards too, their practical experience probably fell a little short of Lupin's, given the many years he had spent struggling at the bottom of society.
But Hermione had actually turned to Jacob.
From what Harry had seen, Jacob might not measure up to the professors at the school, but he certainly had considerable insight into the practical application of magic. He should be able to teach Hermione a few things.
Besides, when a boy and a girl practiced magic together, making progress was quite easy.