The sonographer was asked to do one simple thing and that was to not say the gender. Not exactly difficult. The mother gave her a full explanation on why and told her about the party and such beforehand. The sonographer telling her and giving her a a non-apology saying she was just too excited is a BS reason to have told them.
Regardless of how you feel about gender reveals and her being grateful (hate that f-ing phrase) that she has a healthy baby (no bearing on the situation whatsoever or need to be brought up in the first place), the sonographer was wrong.