How likely is it for someone to get a heart transplant? She is 26 years old and otherwise healthy other than diabetes.

Likely, however... Given the chronic shortage of organs, transplant candidates have to be sorted out very carefully. For a 26 year old many efforts would be made to find a match. It would be up to a heart failure cardiologist to do a full evaluation and determine her eligibility.
In this pt, unlikely. Since you say she is healthy, her diabetes must not be severe. 26 is very young for heart disease, unless she has done serious drugs, smokes/drinks heavily, is obese.Dm pts. Get kidney transplants > heart trnsplnts, & not if no probs!she is young and if "healthy", can actually reverse her diabetes with the proper lifestyle changes:proper nutrition is key, but also exercise&appropriate supplements.