What is the issue they are having? You might need to contact Visible customer service via chat, Twitter message or Facebook messenger.
It should be as simple as clicking the phone the new customer want, click either the box if you want payments or the pay in full box, under trade in the no thanks box should have a blue line around it, click continue. I think at this point it should ask which plan, and then it should take you to the checkout. If they are using your family and friend referral code they need to enter it in the Promo/friend code box and click apply then checkout.
If it is erroring out at this point after selecting phone and plan then they will need to contact customer service.