You are certainly correct that weak social ties can aggravate health problems, but there's no indication that was the case with Nietzsche. The median onset of stroke in people with CADASIL is 50 years[1], but Nietzsche was 53 or 54 when he had his first stroke, so he beat the odds.

[1] http://www.ncbi.nlm.nih.gov/pubmed/15364702