When a person is writing a letter and they're not experienced in writing, is it wrong to find a sample on the internet use it as if it was his own?

All of us learn from the examples set before us, or from the existing wealth of knowledge; that is how human civilization has progressed. There's nothing wrong in using the template or format available on the Internet or in a book. One needs to customize the format to suit one's needs and purpose. But, to use an existing letter as it is and claim it as your own creation, is called plagiarism, though there's no copyright on letters! But still, better avoid using the letter as it is; make the necessary changes to call it your own.   

It is wrong ONLY if you simply copy and paste it. The right way to do it is to read as much as possible samples, then change the content to what makes your letter unique, coherent and complete.

You can use these samples to learn from it how to write a professional letter but not to copy it as it is to avoid the Plagiarism and copy rights issue

It is good to find a sample but it is not good to use the sample as it was. It should be written according to his own requirements. Sample should be used only as a sample not to copy paste. 

That would count as plagiarism, However their is nothing wrong in finding an idea or a suggestion from the internet. It is always better when you put your own thoughts and feelings in what you write.

Well, I guess it is not wrong because he shared it for others to get benefit. the most important thing is that you make sure its the right thing for you. Edit it to fulfill your need. 

Using ideas as inspiration is nothing wrong. However it becomes wrong only if you copy ditto of the one who did it and in my opinion is absolutely illegal. 

Off course  there is lot of stuff available at INTERNET but instead of copy paste it would be much better to learn from it and than type your own there is nothing wrong in learning but i would never recommend copy paste as it is not effective to express your own experience or feeling.

you can use these samples as resources but eventually you will adjust that and add your special touch