If you want to do it online, rather than modify her existing account (like you say they tend to want to upgrade rather than down grade) just open a new account.
If you’re happy with the existing handset just get a sim-only deal with whoever you want, including EE and close the existing one. Even if you stayed with EE they’d probably send you a new sim anyway if you were to go SIM only.
Its a while since I’ve looked but EEs most basic Sim contract would give you varying amounts of calls /texts depending on whether you sign up for a long contract or a rolling month to month one (they give more if you sign up for longer they give more generous allowances for the longer contract). I’m on the month to month one and never use up any allowances though