Well, so much for the quick answer :-) I hope all is well with all involved here.

The victim in question did use the above concoction, along with sitz baths in Epsom Salt.
Couple days and the boil or carbuncle was soft, shrunken, and getting better.

Since nobody else has chimed in, this is a situation for which I can relay anecdotal suggestion from having seen it done first hand in the office of an establidpshment MD ..... ergo, less need to caution "I am not qualified to give medical advice, consult an establishmdnt MD" yada yada yada.

The boil was not quite as large as you described but was located about the same spot on an elderly man. Doc rubbed the boil with some liquid .... probably betadine or similar disinfectant, then opened "lanced" it, then squeezed out most of the gunk, then applied some gel which I could not identify, applied bandage, procedure done and healed well within a week.

Some advise against opening boils for fear of causing it to become more infected than it already is and go the kinder gentler route such as you and friend ..... your call, I suspect this may in part be determined by the constitution of the patient. If it were my boil, I would have sanitized the surface with cayenne tincture, opened it, squeezed out the gunk and then rubbed thoroughly with cayenne & marshmallow tincture soaked gauze then apply a bandage with salve such as Burn & Wound . This past fall I harvested a small crop of roots and now have a gallon of tincture, it does an amazing job of soothing and taking the hurt out of wounds ... I know, alcohol-based tincture sounds counter-intuitive to soothing a wound but it does.

An aspect that astounded me the most about this incident is that it happened 8 years ago, which is to say, when I accompanied the boil victim into Doc's office, I expected him to say something along the lines of " ......mmmmHmmmm, you have a nice boil that once not long ago was child's play for a family MD worthy of the title but these days I must defer to a specialist for minor surgery, thank you, have a nice day, pay the woman at the window"