Can someone enter the usa a one-way ticket

The rule is you must have a return/ongoing ticket.

You will find some people who have gotten lucky with the customs guards, but I wouldn't recommend trying it.

What is your purpose for coming to the USA? Maybe I can advise you better if I know your intentions.

Where are they coming from ?

How long are they staying ?

What type of visa are they coming in with?

Depends on type of visa you have. If you are grad student, or you are on employment visa, you may purchase one way ticket. Other than these the airlines would allow you...