I work with a person from veietnam. they have lived and worked here for years can they legelly take all there money back to there native counrtry ?

Each country has its own rules about how much cash you can bring into the country when you travel there.  I recommend you contact the Vietnamese embassy here in the United States to find out what their limitation is.  The limitation is probably low enough (i.e., $7000) that your friend could not carry everything he or she has earned over many years back to Vietnam as cash.