Ranking depends on a lot of factors. Run a test with Google PageSpeed and see what it tells you, it will be actionable. All of that affects ranking to varying degrees. DA is just one of the factors and high 30s is not necessarily considered a very good or high DA from what I know, it goes up to 100.
Also, there are other factors for rankings such as personalization, demand/market changes in keywords from a few years ago to now in some markets (example: SEO ninja.... SEO Guru.... growth hacker)
And how is he checking on his ranking that he knows is bad? Rankings that moz provides? Google console? his own attempts to search for it?
One last thing, what is their backlink profile like anyway? Those may have been nerfed by Google in their value, but they still count to a degree. There is competition too, he may have found new and aggressive competition... one market like this is photo booth rental market. 3 years ago it was a new market relatively speaking, now it is saturated heavily and there are just too many hands. This will certainly impact ranks.
