Can attacking players use activated abilities after blockers have been declared?
Combat Phase

506.1. The combat phase has five steps, which proceed in order: beginning of combat, declare attackers, declare blockers, combat damage, and end of combat. The declare blockers and combat damage steps are skipped if no creatures are declared as attackers or put onto the battlefield attacking (see rule 508.8). There are two combat damage steps if any attacking or blocking creature has first strike (see rule 702.7) or double strike (see rule 702.4).

Declare Blockers Step

509.4. Fourth, the active player gets priority. (See rule 116, “Timing and Priority.”)

So there is room (for either player) to cast spells or activate abilities after blockers have been declared and before combat damage is being assigned.
But what you describe, &quot;moving her to the command zone&quot; looks like combat damage has already been assigned and dealt. Your opponent can still pump up their creature (in the general case; it looks like it should've already died, as @Arthur noted in the comments), but it won't affect the combat damage already dealt.