Yes -- something sour is the best way to get rid of the taste. Both my kids had to take Flagyl at one time or another, and both detested it. And they were lucky not to get the persistent metallic taste, the way some people do.
A sour sorbet works well because it is both sour and cold. My son liked a sour passionfruit sorbet that we weren't able to find anymore, by the time my daughter had to take the Flagyl. She used Edy's sour lime popsicles.
A rare but serious side effect of Flagyl is a tingling or numbness in the fingers and toes. Keep alert for that one, and let your doctor know immediately, and don't give her another dose until you discuss it with the doctor.