Depends if she likes US cigarettes! If I were you, I'd get your duty free cigarettes at your departure airport to make sure that you get the brand your MIL likes as there are normally only US brands on offer at US airports and the same in shops in Florida.

US cigarettes are much cheaper in the stores than in the UK but they're only US brands and, if do happen to find any UK brand names, they'll be more expensive than the local ones.