I'd go with a mixture of the two, be nice but also point out that your cats have a special diet and feeding them cream etc will make them ill and you are sure she wouldn't want to cause them harm if she is a cat lover.... blah blah blah...
Also, if her cats are obese perhaps she might need reporting to RSPCA?
Some people don't realise that overfeeding and inappropriate diets can be every bit as bad as underfeeding.
If it was her kids or grandkids you were feeding cakes and sweets etc to, she'd soon have a go at you.
Jo XX