Hemingway's use of "benevolent" in "benevolent skin cancer"

It is certainly more common to refer to any tumour or abnormality that is non-cancerous or non-life-threatening as benign, but benign and benevolent do not have the same implication.

Benign implies that the thing being described is of no net detriment.

Benevolent implies that the thing being described is of net benefit.

Though Hemingway undoubtedly knew of the general use of benign, and of course the meanings of the two words, I would suggest that Hemingway was using artistic license in this context in expressing that the blotchy abnormalities in question actively protected the character from the sun's rays (though in reality skin cancer does not protect in this way, and not all changes in skin pigmentation are cancerous). As such they were not merely of no net detriment, but were, instead, of net benefit. Hence the use of the word benevolent.

Being a dermatology healthcare provider and Hemingway lover, I actually think he misused the term cancer rather than benevolent being the word in question.

The description he includes in the text is more aligned with solar lentigos or “sun spots” that are brownish and come from sun exposure, and increase with age and years of exposure. These spots are benign, noncancerous, and and could easily be illustrated as benevolent spots from the sun, gifts from the sun.

It is a literary device akin to 'Antiphrasis'.