billc
The answer to your question is NO.
You have 2 options:
1. Take a second gun case.
a. Use a tuff-pac which will hold the 3 rifles with soft cases and be under 50lbs.
2. Have your friend have all the correct paperwork for the rifle you will be placing in his case. Plus, he has to act like it is his rifle when he is asked. Or you will lose your rifle and your friend will be detained. I an not sure how the prisons are in South Africa.
Ammo is another issue.
The ammo you transport has to match your rifle or rifles and be in a lockable case, plus transported in your luggage.
You cannot put ammo in each others luggage.
You did not indicate the rifle calibers you and your friend are taking. However, Having 2 rifles for the same caliber for one person is also against the law.