Fat experiences

I would say take it slow with her then you can sort of tell her the philosophy of the fetish because the fetish is uniquely different for each person.

Do a lot of non verbal things. Don't say how big her tummy is, but touch it and then just say a lot of nice things about her. Perhaps tell her you like the outfit she is wearing, or the way she smells. Then slowly make comments saying something that her bra is getting small (rather than her body is getting bigger.) By now, if she is still into it, she may like you saying that you like her gaining.