question about digestive issues in a Jack Russel

co-worker has a 6 year old female (spayed) Jack Russel that has constant digestive issues for the past 5 years

diagnosed with giardia at one year of age, and put on prescribed treatment and diet purchased from vet clinic. (sorry, don't know which food)

issues/episodes have been recurring for the past 5 years, off and on, treated by the same clinic (of about 5? vets) she's been scoped, tubed and probed, but nothing found.

Since October dog has been barfing almost daily, and appears to be in considerable pain.

co-worker has noted that when the tummy gurgles are loud, the barfing follows.... fluid vomited up is sometimes clear, sometimes yellow biley, or just consumed food. The most recent treatment (last week) has suggested food alergy, as the culprit and prescribed different food. No change in the barfing.

Dog has cat companion during the day, but with no access to cat's litter box.....

Co-worker is at the end of her rope, any one have any ideas?

comments, thoughts appreciated

She is collecting up her dog's medical reports and is making an appointment for a second opinion with my vet.