The new Apple Watch Series 3 comes with watchOS 6 which will NOT pair with an iPhone 6 Plus. And the iPhone 6 Plus cannot upgrade to iOS 13, which is required for watchOS 6.
Return the watch immediately, as you have 14 days to get a full refund.
You can always buy a new Apple Watch later when the iPhone model situation changes.
iPhones that will work with the Apple Watch running watchOS 6 are the 6s, SE, 7, 8, X, XS, XR, 11 and 11 Pro
Again return the Apple Watch and get a full refund, then work out any issues later with all your money in your pocket.
A used iPhone 6s Plus would be the least expensive upgrade for your relative, but only consider that after getting your money back for the Apple Watch.