Oooo! Yet
another fake grammar rule!
There are miles between 'minimise your use of adverbs' and 'don't end sentences with adverbs.' The first is good advice, the second is nothing less than a fake rule that someone made up.
Seriously. A simple fix to accommodate your friend's fake rule would be:
"I do not agree," Missy stiffly says.
Grammatically, this sentence is fine; it also abides by your friend's fake rule. Does that make it an effective sentence? Methinks rather not!
So. 'Minimise adverbs'? Yes. 'Don't end sentences with adverbs'? No.
Need confirmation? Open the books on your shelves and skim them for examples. I guarantee that you'll find sentences that end with adverbs.
Also? IMO, your original sentence is perfectly fine. Removing the adverb changes the meaning, and any other alternative adds words without any real benefit. If you've already minimised your use of adverbs, this one instance isn't a problem.