I know plumbers than can change them on a full tank all the time and if Robin has experience changing elements then she could probably do it also so I see nothing wrong with Jack's advice to her. She was advised she could get a big belch of water. However she indicated that she got the water to drain by opening a faucet so that apparently isn't an issue here anyway. Since she can change an element but not know about electric water heaters having upper and lower elements along with thermostats and how to test them could be a problem though.
We could also point out that those plastic drain valves have a habit of breaking, cracking, leaking, and not shutting off completely.
Guess I'll have to talk to a shrink about my sanity but he/she would probably tell me to stay off the the internet.