Mini lessons with the teacher are OK. If they are with a sub, you are not following the IEP and this opens the school up to a due process suit. Subs cannot instruct this student or they are not following the IEP. With the IEP, the school & family agreed to this. It is a binding contract that the school should be following. If you want to find balance, you need to redo that part of the IEP.
When my sons were in 3rd grade, their teacher was on maternity leave. They started the year with a sub. Their teacher was due to come back for the last few weeks of school but the parents advocated that she not come back to her classroom so the sub they had from Sept - May could finish the year with her class. If this was the situation, this child would miss a full year of instruction. My twins also had an English teacher in 7th who went on sabbatical for 1/2 the year where they had a sub. Long Term subs aren't like a daily sub, maybe the IEP should have covered this happening.
It's a situation where you are between a rock & a hard place. Do your job & teach this student so they can learn which violates the IEP or follow the IEP & don't provide instruction for a months long maternity leave and that verges on educational neglect. Teachers are not paid enough to make this decision themself. It really requires admin to figure this out.
Do you know the backstory on why this is in the IEP? Did a sub contradict what the teacher said which resulted in a shutdown of the student? Knowing this might help the IEP team come up with something appropriate to the situation.