I think a probiotic would be a great idea. Buprenex is more likely to cause constipation than diarrhea, but individual cats may have different reactions. Was Boo also vaccinated at the same time? Sometimes the cumulative stress of all that going on in their little bodies can cause GI disturbances.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler