Not very familiar with the libraries, and BB_C has already given a lot of more detailed feedback, but here are some small things:

• Even without rewriting get_files_in_dir as a single chain of method calls, you can still replace

  if dir.is_err() {  
      return None  
  };
  // use dir.unwrap()
  

  with

  let Ok(dir) = dir else {
      return None;
  };
  // use dir
  

  In general, if you can use if let or let else to pattern match, prefer that over unwrapping -- Clippy has a lint relating to this, actually.
• Although the formatting doesn't seem horrible, it doesn't seem like Rustfmt was used on it. You might want to make use of it for slightly more consistent formatting. I mainly noticed the lack of trailing commas, and the lack of semicolons after return None in get_files_in_dir.

Let's do this incrementally, shall we?

First, let's make get_files_in_dir() idiomatic. We will get back to errors later.

fn get_files_in_dir(dir: &str) -> Option<Vec<PathBuf>> {
    fs::read_dir(dir)
        .ok()?
        .map(|res| res.map(|e| e.path()))
        .collect::<Result<Vec<_>, _>>()
        .ok()
}

Now, in read_parquet_dir(), if the unwraps stem from confidence that we will never get errors, then we can confidently ignore them (we will get back to the errors later).

fn read_parquet_dir(entries: &Vec<String>) ->  impl Iterator<Item = record::Row> {
    // ignore all errors
    entries.iter()
        .cloned()
        .filter_map(|p| SerializedFileReader::try_from(p).ok())
        .flat_map(|r| r.into_iter())
        .filter_map(|r| r.ok())
}

Now, let's go back to get_files_in_dir(), and not ignore errors.

fn get_files_in_dir(dir: &str) -> Result<Vec<PathBuf>, io::Error>
{
    fs::read_dir(dir)?
        .map(|res| res.map(|e| e.path()))
        .collect::<Result<Vec<_>, _>>()
}

 
 fn main() -> Result<(), io::Error> {
     let args = Args::parse();
-    let entries = match get_files_in_dir(&args.dir)
-    {
-        Some(entries) => entries,
-        None => return Ok(())
-    };
-
+    let entries = get_files_in_dir(&args.dir)?;
 
     let mut wtr = WriterBuilder::new().from_writer(io::stdout());
     for (idx, row) in read_parquet_dir(&entries.iter().map(|p| p.display().to_string()).collect()).enumerate() {

Now, SerializedFileReader::try_from() is implemented for &Path, and PathBuf derefs to &Path. So your dance of converting to display then to string (which is lossy btw) is not needed.

While we're at it, let's use a slice instead of &Vec<_> in the signature (clippy would tell you about this if you have it set up with rust-analyzer).

fn read_parquet_dir(entries: &[PathBuf]) ->  impl Iterator<Item = record::Row> {
    // ignore all errors
    entries.iter()
        .filter_map(|p| SerializedFileReader::try_from(&**p).ok())
        .flat_map(|r| r.into_iter())
        .filter_map(|r| r.ok())
}

     let entries = get_files_in_dir(&args.dir)?;
 
     let mut wtr = WriterBuilder::new().from_writer(io::stdout());
-    for (idx, row) in read_parquet_dir(&entries.iter().map(|p| p.display().to_string()).collect()).enumerate() {
+    for (idx, row) in read_parquet_dir(&entries).enumerate() {
         let values: Vec<String> = row.get_column_iter().map(|(_column, value)| value.to_string()).collect();
         if idx == 0 {
             wtr.serialize(row.get_column_iter().map(|(column, _value)| column.to_string()).collect::<Vec<String>>())?;

Now let's see what we can do about not ignoring errors in read_parquet_dir().

Approach 1: Save intermediate reader results

This consumes all readers before getting further. So, it's a behavioral change. The signature may also scare some people 😉

fn read_parquet_dir(entries: &Vec<PathBuf>) ->  Result<impl Iterator<Item = Result<record::Row, ParquetError>>, ParquetError> {
    Ok(entries
        .iter()
        .map(|p| SerializedFileReader::try_from(&**p))
        .collect::<Result<Vec<_>, _>>()?
        .into_iter()
        .flat_map(|r| r.into_iter()))
}

Approach 2: Wrapper iterator type

How can we combine errors from readers with flat record results?

This is how.

enum ErrorOrRows {
    Error(Option<ParquetError>),
    Rows(record::reader::RowIter<'static>)
}

impl Iterator for ErrorOrRows {
    type Item = Result<record::Row, ParquetError>;

    fn next(&mut self) -> Option<Self::Item> {
        match self {
            Self::Error(e_opt) => e_opt.take().map(Err),
            Self::Rows(row_iter) => row_iter.next(),
        }
    }
}

fn read_parquet_dir(entries: &[PathBuf]) ->  impl Iterator<Item = Result<record::Row, ParquetError>>
{
    entries
        .iter()
        .flat_map(|p| match  SerializedFileReader::try_from(&**p) {
            Err(e) => ErrorOrRows::Error(Some(e)),
            Ok(sr) => ErrorOrRows::Rows(sr.into_iter()),
        })
}

 
     let mut wtr = WriterBuilder::new().from_writer(io::stdout());
     for (idx, row) in read_parquet_dir(&entries).enumerate() {
+        let row = row?;
         let values: Vec<String> = row.get_column_iter().map(|(_column, value)| value.to_string()).collect();
         if idx == 0 {
             wtr.serialize(row.get_column_iter().map(|(column, _value)| column.to_string()).collect::<Vec<String>>())?;

Approach 3 (bonus): Using unstable #![feature(gen_blocks)]

fn read_parquet_dir(entries: &[PathBuf]) ->  impl Iterator<Item = Result<record::Row, ParquetError>> {
    gen move {
        for p in entries {
            match SerializedFileReader::try_from(&**p) {
                Err(e) => yield Err(e),
                Ok(sr) => for row_res in sr { yield row_res; }
            }
        }
    }
}

Your usage and implementation of get_files_in_dir is quite awkward. It is returning None on errors and then you're returning Ok(()) when it returns None, ignoring a bunch of errors. Why not just have get_files_in_dir return a Result<Vec<PathBuf>, io::Error>? You could drastically simplify the function by just ?ing the results.