A quick glance at Queen Elizabeth’s former equerries reveals that … yes, they end up as Group Captains, Squadron Leaders, Vice Admirals etc. I’d expect Major Johnny gets promoted Lieutenant Colonel and either command of a battalion of his regiment (technically Balaklava Company is a battalion but it’s very small!) or some kind of staff officer position.
I think I know who you mean; I don’t believe she’s a military equerry since she is never in uniform. I think she’s Camilla’s lady in waiting or else works for Charle’s comms team. It would be major (pardon the pun) news and widely reported for the royals to have a female equerry, just as it was when the Queen appointed Lt Colonel Nana Kofi Twumasi-Ankrah as her first Black equerry.