There are tons of threads on the various Android forum sites regarding battery life. I strongly recommend searching for them and referring to them as my memory isn't 100% reliable.
Definitely start with the Battery Info screen and tackle the battery hogs. Most people don't realize that their usage can greatly impact battery life. If you're browsing for hours on 3G there's little to be done about it -- you're going to use up the battery. Any new device goes through a "new toy" period where the user uses it much more often than normal and battery life is typically very poor during this period.
If WiFi is available, try to use it as WiFi uses a bit less power than 3G for the same amount of data.
There are things such as GPS that can impact battery life but the GPS receiver only eats the battery while it is actively used. If an app frequently polls the GPS receiver for location that can impact battery life. You can turn off GPS but I'd recommend addressing the actual problem and either adjusting the polling settings or whatever it takes to deal with the problem app itself. Some apps don't seem to properly release the GPS receiver which can also cause battery life issues.
Poor coverage will definitely impact battery life.
Frequently polling for email or other data will also kill your battery faster.
The display is generally a battery hog for everyone, including myself (and in general my battery life is very good).
She is already using app killer and just got the phone....maybe 2 weeks ago so I would think that life would be pretty good right now.
Do not use an app killer to improve battery life. Odds are that whoever recommended this didn't know what he/she was talking about. Unless you really know what you're doing, remove the app killer. You can actually make battery life worse by killing the wrong apps.
Again, consult the other threads for more suggestions.