Juba, you don't really need antibiotics to cure your friend. I was in the same situation 2 days ago, caught a cold while playing at night, and I thought I was dead for sure. Then the next day I headed to Cherno's hospital desperately, trying to find these so called antibiotics. On my way to the hospital I finally found some matches and wood to restore my temperature, and when I finally hit Cherno's outskirts I noticed that my survivor was no longer coughing. I ate some cooked meats to increase my blood beyond 10k to check if I'm still losing blood because of the sickness. Surprisingly for me, I was healthy again, so I would advise you to light a fire instead of searching for that really rare antibiotics, and keep your friend's temperature 90+ for some hours, and he will be cured. At least it worked for me.
