There are many factors which can contribute to male sterility, and if your friend is concerned about this issue, he should definitely have a full consultation with a urologist
or with a fertility expert. Many causes of male infertility
can be identified, and some of them are treatable.
I do have some doubts that fever alone could permanently cause infertility. While it is true that high temperature decrease sperm viability, this association is weak. Furthermore, it would be unlikely that fever alone could permanently damage the ability of the testicle to produce sperm.
There are some exceptions to this. For example certain viral infections, especially mumps, may cause high fevers and also at the same time swelling and inflammation in the testicles. Mumps is one of the leading infectious causes of infertility worldwide.
It is also possible to have infertility from a serious medical illness, often due to the malnutrition that results or from shock of the illness to the pituitary gland, resulting in decreased amounts of sex hormones. Sometimes this damage may be permanent, if the episode of illness was severe enough to permanently damage the pituitary gland.
Your friend should talk to his doctor
for more advice on this issue.