How to Get Rid of Chin Fat for a Thin Person?

 There are 2 locations of fat present in the neck, and these are sub-platysmal and sub-mental fat. Both can be removed with a neck lift without any skin removal. A platysma plasty will be required  in the front portion of the muscle once the sub-platysmal fat has been  removed.  Digital computer imaging may be of some benefit to see what her new neck might look like. Please see the link below for examples of neck lifts in our practice

From your photo, you appear to have a 'small jaw' appearance which would include the width of the jaw as well as the profile appearance. The distance from the 'neck/adam's apple' area to the chin point; is short and so the area under the neck appears full.

More images are necessary to review, but you could benefit from either an orthognathic procedure for jaw lengthening; or masking with jawline augmentation with or without treatment of the tissue under the neck (depends on laxity of the tissue/age etc)