Could I ask if you think this will help with his SEO rankings.
1 backlink will hardly make any difference, especially in the automotive niche which is not easily approachable. If I were to blind-guess I'd say that THAT particular guest post won't make any difference if you point the backlinks within to the homepage of your grandfather's site and the homepage targets competitive keywords (by competitive, in this case, I'm referring to keywords with 1000s of searches per month, although that's not the true definition of "competitive keyword".)
Now, if you do point the backlink(s) within that guest post to a content page that targets a long tail keyword (or a cluster of closely related long tail keywords) that's ranking on page 2 of google, for example, then that guest post might just happen to propel that page to google's page 1 if your grampa's domain is decent (well structured, fast loading site, no technical SEO issues, good topical authority, etc.)
But notice the use of the "might just happen to" words because this is as close as I can estimate of how well that lone guest post will fare for your SEO based on the little info we have on your grampa's site and on its competition. SEO is not exact math, as in my competitors have 2 links therefore I can beat them with 3 links of the same quality, and the reason for this is that google looks at over 200 ranking signals before deciding where each URL should rank and for what terms. And sure, backlinking is still one of top 10 (arguably top 3) ranking factors nowadays, but it's still not the be all end all solution to SEO, so...
