The engineer got a pay rise and no bonus.
Both do two completely different jobs and it's completely idiotic to even compare remuneration against each other.
The hamper's to me are a nice gesture, but really quite irrelevant.
If the bookkeeper wants to negotiate her pay - then absolutely go for it. But pick an appropriate time/place and don't have a hissy fit about it.
What's happened is the boss (OP's DH) has gone to his bookkeeper and said "hey I want you pay $x.xx amount of bonuses to all of these people except for Sally."
The bookkeeper has said "oh why isn't Sally getting a bonus?"
The boss has said "because I've arranged with her that in lieu of a bonus she'll be getting a payrise next year".
The book keeper has then chucked a massive tanty because Sally is getting a pay rise.
Unless the book keeper was genuinely asking why Sally wasn't getting a bonus thinking that perhaps she had forgotten - it was really none of her business to ask.