Wouldn't "caiff hi" be "she may"? Doesn't the mutation turn it into a question... "may she?"
Sometimes, but not always. A statement starting with caiff etc can be mutated, as it often is with forms of cael and gwneud particularly.
This comes from a missing particle mi or fe which you will sometimes hear in front of positive statements.
- Mi gaiff hi goffi, wrth gwrs - She may have a coffee, of course
- Fe gaiff hi goffi - She may have a coffee
- Gaiff hi goffi - She may have a coffee
- Caiff hi goffi - She may have a coffee
A question form will always be mutated, be followed by a question mark in writing and, as in English, have a rising tone in the spoken question.
- Gaiff hi goffi? - May she have a coffee?
(That mutation at the beginning is also caused by a different missing particle which used in front of questions, but one that you will not come across in informal Welsh. )