Short answer
A few spoonfuls (about 1–2 tablespoons) of plain kefir per day is generally safe for a dog of that size. Give it plain, without any sweeteners or flavorings. Watch for any stomach upset such as vomiting or diarrhea. If you notice those signs, call your veterinarian.
Kefir — Food safety facts
Feed with care · Dogs · full breakdown on the linked food page
A small amount of plain kefir can help some dogs. Never the flavoured, sweetened kind.
The full verdict, portion guidance and serving notes are collected on the Kefir food facts page.