They should be able to enter the most recent validation code sent to them if within a few minutes. If they are not sure which one that is, I suggest they clear their text messages from the service, refresh the screen and try again. Once they enter the last of the 4 digits they do not need to hit ENTER, they just need to wait a second and the code will come.
They should be able to hit refresh in the browser to reset the page. Ideally they should wait a minute or two before trying again. Make sure they are entering the verification code in the correct field and clicking on the view documents button to submit. It may be they are hitting enter and that may be resending the code instead of opening the document.