Interaction between facebook content and joomla (vmart)

[Fintan]

Hello, I have a client who has a fashion store. His facebook page is very well received and he runs weekly specials on one or two products. What he would like to do is when a visitor on his facebook sees a special that she likes she can click on a link that will take her to his joomla site and log her in with her facebook account  and place the product into the vmart cart. From here she can make her purchase.

Any pointers are greatly appreciated Thanks