Since he is in valley I think it makes sense for him to learn Core Java with test automation tools like Selenium. What are your thoughts?
Java, OTOH isn't widely used in startup shops. It's strengths are in very large codebases with many developers. To achieve those strengths, a lot of overhead is necessary. That overhead is too much for small shops with few developers. They are going to be 10x more productive in a scripting language. Also, as high quality/proven correct code is not normally necessary in tests the overhead time will be wasted. Of course if their main product is in Java or C++ for example, many of the tests should be too. But unit tests should be written by the original dev.
Selenium has bindings in most languages these days, so few limits there.
The industry changes fast enough that I wouldn't worry much about picking the perfect longest-lasting language that you are going to take with you into retirement. It could be replaced in as short as five years, and very likely to be in ten.