How can my friend help his cat?

Sounds like nothing, if the cat won’t let him examine the leg AND your country doesn’t have vets. What about trying to get advice from a regular doctor, rather than a vet?

And what country is this, if I may ask?

If there is no visible wound and the cat is putting weight on the leg it may just be a sprain from a bad jump/landing or fall from something. If there is a wound somewhere on the leg, a bite maybe, it may be abcessed and infected in which case it would need to be kept draining and an antibiotic would help, but only one prescribed by a vet.
Not much they can do if the cat will not let them examine the leg. Just keep an eye on the cat and hopefully your friend is feeding it well too.

Your friend needs to inspect for any sort of cuts or puncture wounds, if there are any sort of cuts he can put antibiotic ointment on the infected areas. If there are no cuts, scraps or puncture wounds then the cat must have hurt it somewhere possibly breaking the bone. This would have to heal on its own if he is unable to keep a splint on the cat.

If there is an infected wound, and you can wrap the cat in a towel while exposing the wound (and not get sliced to shreds), clean the wound with Normal Saline (or sterilized water by boiling it. Hydrogen Peroxide is OK too, in a pinch, just pour it on.)—soaked sterile or clean 4×4 gauze pads (or boiled rags) and dab the wound gently to clean it. Coat the wound with a thin coat of Silverdiazine (Safe for cats, birds, dogs and horses.) paste or powder. Human Triple antibiotic ointment or Neosporin ointment is safe too, but Silverdiazine is much better and has been around forever. If it is a large wound, cover with 4×4s and tape, but you’ll have to shave the area first. Repeat daily, but stop using Hydrogen Peroxide once you see the wound is no longer infected (it inhibits new skin growth) use normal saline or sterile water in its place. If your friend lives near a town, there should be a pharmacy/farmacia/apothecary/apotek/doctor/medico or medical person where he can get these very basic items. Wear ear plugs~. Monitor for lethargy and check the cat’s temp 2x daily to Monitor for systemic infection. Normal Range is 100.5F—102.5F. Make fresh, clean water available at all times; flavor with juice from a can of fish or blood from the real thing if cat is reluctant to drink. Use a cooking syringe to get it down if necessary, but just put its in is mouth; don’t shove it down its throat. Search the net for FIELD-EXPEDIENT VETERANARY MEDICINE. Look for cats/wounds/treatment and systemic infections/treatment.

Thanks everyone for your advice I will pass it on to my friend. And hopefully the cat will get better