I’m so sorry that you’re going through this and I know how tough it can be when our relatives are sick. I don’t see an issue with performing a healing spell on someone to help them.
Since you’re not doing the spell on yourself, you can try this spell and adapt it using a poppet of your father instead. Give the poppet the bath instead of yourself 
@SilverBear also posted this healing spell in the forum that you might want to have a look at.
I wish you all the luck in your spellcasting and I pray that your father is on the mend soon 