This is certainly a tough question. If you were to ask which would be the better option, I would say go with the second one, because you will have no affect if the parents can't keep her on her regimen. At least in that situation she would be getting some form of treatment, and it is typically better to remain consistent with one medication due to the effects of switching medication constantly.
The more discriminatory of the two options, however, would probably be to maintain that single medication. Typically, if the treatment plan goes properly, there is an understanding that the parents of a child will maintain the treatment plan appropriately to take care of their child. If that were the case, you would be able to find the perfect medication, taking time to test various ones so that you could treat the child best. However, since this is not the case, you wouldn't be able to take that action.
Leaving Lia on one "sufficient" medication is more discriminatory because you typically would pursue further treatment and better medication options if you had the participation of the parents. Unfortunately, this act of presumed discrimination is out of Dr. Ernst's hands because he can't control what the parents do; he is only responsible for ensuring Lia gets the best treatment possible.