My wife & I have an 18-year-old son. He started university in September, studying medicine. There he met this 41-year-old man - a classmate, and they became really good friends. This man has just started university now at this age because apparently he was born poor and in the first half of his life was focused on making money, but medicine is a dream of his. Son has always been an introverted, shy, socially awkward kid with little friends, but now goes out often. Honestly my wife & I are uncomfortable, and we can’t help but side eye the dude.
What, specifically, are you concerned they might do, or might become? Are you afraid this might become a predatory sexual relationship? That he might become codependent? That the older guy is immature and will be a bad influence?
When you can define your concerns, it will reveal what action (if any) is appropriate.