Have your coworker go to the Apple store and pay cash for the iPad.
That way you avoid all the problems with trying to use a foreign
credit card - some stores will not accept them. If your coworker
has a US credit/debit card, he could use that instead of cash.
To have iPad shipped to his relative's house, you will need US
credit/debit card with US billling address. I am not sure that you
will be able to place the order from outside the US using those items,
but coworker's relative could place the order for you.
If iPads are not yet for sale in your country, you will have to take
the iPad to a country where it is sold if there are problems requiring
repair/replacement. You cannot ship it to Apple US for repair/replacement.