That guy and wife were really scared, and I lost touch with them so don't know how things turned out.
Googled - consensus is usually doesn't cause sterility but can cause difficulty fathering a child, doesn't say it will, always, etc.
If a male gets mumps, one or both sides is irrelevant, and if that person stays in bed, and does not do any strenuous exercise for at least a week, or until the mumps go away, his maleness will not be affected.
But, if that male with the mumps DOES NOT stay in bed, but instead, engages in strenuous exertion, the mumps will migrate to his scrotum; and THAT is what causes the sterility.
I got the mumps at 25 years of age. The doctor told me to go home, go to bed and stay there for at least a week. Instead I went to a friend’s horse farm and rode Cutting Horses. By sundown the mumps had migrated south, and at that point I DID land in bed, with ice packs in a strange place. Fortunately, my wife was 8 months pregnant with our first and only child. And I’ve been firing blanks all these years since then........certified by our family physician.