What has the vet advised. Having had Giarda myself on a number of occasions while backpacking I feel for the poor thing (I had exploding everything from everywhere and couldn't keep anything down or in!). I was put on flagyl (metronidazole), which is a potent antibiotic also used for campylobacter but don't know what the doggy version is.
I would also enquire as to whether you could catch it as well as I believe the bug is the same in most domestic mammals (excl. rodents). This was from the Merck vet manual (manual also says its quite common):
"Giardia cysts survive in the environment and thus are a source of infection and reinfection for animals, particularly those in crowded conditions (eg, kennels and catteries). Prompt removal of feces from cages, runs, and yards will limit environmental contamination. Cysts are inactivated by most quaternary ammonium compounds, household bleach (1:32 or 1:16 dilution), steam, and boiling water. Cysts are susceptible to desiccation, and areas should be allowed to dry thoroughly after cleaning. Cysts contaminating the hair of dogs and cats may be a source of reinfection"
Also like to add that giardia is something that can be caught in th eUK and you don't need to go abroad to get it in humans.