You are confusing me, lady. Your boyfriend bought lipo batteries, these are the best battery tech you will find, much lower weight for the same capacity, and they don't slow down the truck as they discharge (they keep voltage better). You said he also bought a charger and an adapter, so all you need now is a low voltage alarm because it looks like the standard ESC is crap and probably doesn't have it. It's like a $2 part, stick it somewhere with some velcro or double sided tape and plug the lipo balance lead into it. Stop driving when it beeps at you. You certainly can just buy a bigger NiMH battery, but it will be twice as heavy if it's got twice the capacity.
I highly recommend reading up a little about this hobby. It will take you at least a few hours before you're able to understand every post in this thread, and to know what you can and cannot do with your trucks.