I wonder if your friend occasionally has jills in season and that is filling your boys with sexual tension?
Occasionally, competition for food can create fighting too.
Make sure your pal changes his clothes etc after handling his jills and reassess your feeding.
You might also want to consider the use of an implant, called Suprelorin, which chemically castrates hobs without the risk of surgery or of them developing Cushing’s Disease.
I have mentioned its use in jills before but it is also useful (and licenced for use) in males for suppressing fertility, decreasing odour, urinary marking and reducing territorial aggressive behaviour.
The implant requires sedation or anaesthesia but it lasts for up to four years.
Your vet will advise further and should be able to provide some decent literature on looking after ferrets.