Just another example of this UK measuring system "over hyping/false advertising"
Back off 4 cups sizes and you get a US D, add 4 inches to the rib cage and you get a US 32 inch band size. So she is an only slighty above average US 32D and if you figure that she is pushing things a cup size by claiming FF rather than F then she is as was said above a totally average US 32C.
But 28FF "sounds" so much bigger
First learn something about bra sizes and the UK vs. US system
. She is a 28H in US sizing. This translates to a 30G, 32F in US sizing, so a '32DDD'. This a 32E here in the UK. Furthermore, you don't add 4 inches to get your band size, your ribcage measurement is your band size, rounded up to an even number.
So far the off-topic part, on-topic, I hightly doubt her breasts weigh so much to become a problem for her partner during lifts, but she isn't flatchested.
xx