Forum Moderators: not2easy
What you need to get is a writer's agreement (find a lawyer or search the web for examples), and that will give you protection in the event the writer sells or gives you rights they do not have.
But, just because the writer's work appears on the other site does not necessarily mean they have lost the rights to it. They may have sold only first rights or non-exclusive rights, etc.
Unless a writer sells all of their rights, they are free to market their works elsewhere, subject to the agreement(s) they've signed with sites they've sold to.
So, you need more than just their name appearing with the article, you need an agreement to protect yourself against any claims arising from your use of their work.
If the person emails you the article, print it out with all the headers. This will at least provide you with some evidence that you did not steal it.