Yes, you can. But in geneal you don't want to. Unlike the BB, or PalmOS, or even Winmob, Android, especially 2.0 and later, does a very good job of managing memory and cpu. The fact that an inactive app is sitting in memory is rarely a problem. If an active app needs that memory, or you launch a new app, Android will itself close the inactive app and make the memory available. Otherwise, Android leaves recently used apps in memory so that if you need it again, it's available immediately. Android will also, under some circumstances, load an app or service that's related to a running app, in case it's needed. All of this makes the OS faster to respond in most situations.
The BB OS, if I understand correctly, isn't capable of this. If memory is assigned to an app, it's unavailable to any other app, so you need to kill unused apps. Not so in Android.
If you close an app or service, and then need it again, it has to be reloaded, which takes time and uses MORE power than just leaving it sitting there would use.
Remember, empty memory uses no less power than used memory. If it's RAM, it still gets refreshed constantly. If it's ROM or NAND memory, it only uses power when being read or written. So an app loaded into memory only uses power if it's doing something.
Here's a more detailed explanation of how Android manages memory:
http://www.androidcentral.com/fine-tuning-minfree-settings-improving-androids-multi-tasking
Now then, to answer your question: Yes, you can manually kill apps with or without an app killer program. On the EVO (and I assume an X is very similar), from the home screen press menu, then settings, then Applications. Wait for it to load the list of all apps. You can then tap any app and, if it's actually running, there will be a highlighted button labelled "Force Close."
The free application System Panel does a good job of showing how background application use (or don't use) system resources. Spare Parts does a good job of showing what applications are actually using battery. They'll show you that leaving unused apps in memory really doesn't impact battery life.
Now then, task killers. If you use a task killer manually, it's not much different that using the Applications list in Settings, although perhaps more convenient. You just need to be careful to kill only apps you want to stop, not everything that's in the background. Why? Because many background processes are needed by other apps, and closing needed processes can cause problems. Some people swear by task killers, others swear at them. For everyone who says something like "using ATK on auto gave me great battery life!" there's someone else who says "since I stopped using ATK my system is much more stable."
Before I got my Evo I thought I'd need an ATK to improve battery life. But when I actually studied how Android works, and looked at the resources background apps use (or don't use) I decided against it.
I easily get 15 hours of battery life with my Evo, and would consider myself a moderate user. Today, after 9 1/2 hours of use, I had 45% left. If your buddy really wants to figure out why his battery life is bad, he should load up the apps I mentioned above and see what's causing the problem. An automated task killer is just a bandaid, not a solution.