In terms of hard core scientific evidence the jury is out. More accurately I’m not sure there is even a jury, as in a decent size well controlled trial.
Anyway if it works for your husband’s pain use it.
There is no doubt it has an active ingredient (taken orally it causes diarrhoea) so given it is an oil, and with the application of heat at least a small quantity will get into your system.
As for what that might do. If you have a look at this:
https://www.ncbi.nlm.nih.gov/m/pubmed/19041719/
There is literature showing castor oil inhibits cytochrome P450 3A which is otherwise know as CYPD3A and the CYPD3A4 variant metabolises daclatasvir so inhibiting it with castor oil might theoretically cause higher daclatasvir levels.
That is mega speculative but possibly as good a guess as you will find.