If you're talking about practicing Jill's song for the Episode 8 performance, this is not true (unless DPC patched it after 0.8.2). You can absolutely practice the song even if you never make a move on Jill (beyond the required tennis date).
You need to practice the song at least twice (I think), and you have three opportunities to do so: once in Episode 4 (in Derek's room, Sage's room, or Bella's house depending on where the MC is staying), once in Episode 6 (during the first free roam in the MC's new room), and once in Episode 8 (again, the first free roam in the MC's room). There are other times when the guitar is clickable, but the MC won't feel like practicing then for reasons. You have to practice during those three opportunities, but they are not gated otherwise.
View attachment 4650516
View attachment 4650517
[Sadly, the new phone obscures the whole screen in Episode 8, but take my word for it, that's the MC practicing in Episode 8.]
As long as you get the practice in, the MC can play at the concert and help Jill get past her stage fright regardless of whether he's on her path. There will be more interaction with her if he is on her path, though.