Yea I'm that friend If you do have to store your batteries and dont have something like a battery safe (check these guys out: https://www.bat-safe.com/) then the oven will contain most fires, though it may ruin it in the process. Do not store your batteries outside or in the garage, you will have an issue at some point.
This is one of the reasons I'm so paranoid about lipos and charging. I've had a few friends have fires, I've had a few myself (luckily not the battery's fault but the electronics it was connected to), and I've watched people lose a ton of money in electronics as we watch the fireball rage. I love me a lipo personally and use them all the time, however a lot of care must go into them if you want to be safe with it. The battery safe I posted above actually is rated to both charge and store batteries in it and even has a filter to help tone down the fumes if you do get a flame up. My buddy also make a homemade storage box too real cheap, I'll get a picture from him tomorrow so we can copy it if needed.
I'm really glad it was something minor and that he lost nothing of substantial value (family, pets, etc), I enjoy his work and look forward to seeing more!