Yes you can become a good one if you have a good idea, a good premise, control the narrative well, and edit your story well. It is all in the idea and the execution of it.
Yes, a bad writer can become a good one.
And a good writer can lose his/her MOJO.
There are also a lot of adequate writers who are successful because they do the work.
I believe brilliant writers have a talent that can't be learned. Crappy writers can practice and improve, but there's more to capturing a character or crafting a plot than can be learned in a book. It's intuition and seeing the world in a unique way.
If he starts researching how to construct a story and work at his writing craft, he gets better.
We ALL start out as "crappy writers." If we persist, and keep writing, we improve. The trick is not to get to discouraged because your first few attempts do not meet your expectation. Keep writing and you WILL improve.
Everyone can improve their writing ability. It is never too late to develop new and improved skills.
Can a crappy piano player become a decent one? In most cases, yes, so long as the person is dedicated, diligent, and patient. Will that person be the next Chopin if they really put their mind to it? Well, there's certainly no guarantee, but theoretically, it's possible. If a crappy writer were to read 500 books over the course of five years and write every day, setting benchmarks and achievable goals along the way, it's essentially a guarantee that he or she would see some degree of improvement. Would that person be well on his or her way to becoming the next James Joyce? Impossible to say. But there's a lot more to writing than just innate ability. It's a learned skill that needs to be honed over time.
that is his lot in life
Yes. One baby step at a time.
Some people who are bad at something can become good at it with a lot of practice and the right guidance.