I would suggest that all of you first try and understand that all ailments and disabilities that would require a service animal are not always visible. The ADA was set up for this and many other reasons. You have no way of knowing not only the disability but the manner in which the dog was trained to assist the individual, nor is it proper for you to inquire anything more than what it was trained for. Assuming anything in this instance is jumping to a conclusion that most likely is wrong. Should something ever happen to you where you need a service animal of any size, shape or breed, you would not be so quick to judgement.
Just to let you know, my wife travels with a small service dog who has done wonders for her health and well being and has given her a new lease on life. Her disability is not evident, is chronic so it won't go away. You'd be surprised at the questions she receives. She is gracious, always responding kindly. I would not wish what she has on any others so I support and assist her wherever possible and explain to others the benefits her service dog provides to her. You might consider the same.