It's a classic!
You see, Airwasp, that's one of the classic cases where your staff member and his seat neighbour, being brought up in different cultures, object to different behaviour. Most people I know would have nothing against either breast-feeding or drinking.
On any normal day, my own reaction would have been to ask why the gentleman objected to my enjoying a g&t. If in particularly high spirits, I might have tried to take a lighter note and ask if he felt the particular brand of tonic was objectionable.
In any case, I think there's no ideal response. It was a strangely impolite remark from someone you have a right to expect politeness from. Best case, your staff member might have started a deeply philosophical and spirited conversation about cultural and religous standards with his neighbour. It has happened to me in similar circumstances.