Two males who aren't in contact with females can go a bit lairy if they are in sniffing distance of sows. Though most Rescues/breeders/boardings will keep boars/sows in proximity.
Your friend can try the complete start from scratch re-introduction (once her boy is healed , poor piggies)
Everything neutral. Cage, bedding, bath them.
Put in a neutral territory and watch them. (But if it escalates to intervene. There's 'levels' of alert depending on their body language)
They might get along fine, they might not.
Even if they don't , they can still side-by-side,
My GP6 was a Rescue who fought with his brother (both were neutered and rehomed)
My GP1/GP2 went through a scrappy phase - we kept them apart at night (winter night cage) but they were together by day (more room in the Pighouse) and they were fine. By the next winter, they were ok together,
Boars are finicky. Some get on , some don't.
Mine went to boarding and clucked like chickens at the sows next door.
How are your sows behaving? My 2 are being right little gits (we have 2 sows+ neutered boar) I think it's a full moon, and they're in competition for GP6.