If the lock system and keypad both have RJ12 connectors, then there is something else going on.
I can not think of a single manufacture ever in 26+ years, that would have RJ12 connections at both ends, but would require you to splice a heavy gauge wire in for a short run. No, no, no... something else is going on.
There is a problem somewhere and it is NOT wire size related.

If you are going to help your relative out, you are going to have to get the make and model of everything involved and do some research. If you can't find copies of the installation instructions online, then the locksmith HAS to supply you with them.
After you have familiarized yourself with the system, you will need to go thru it and check every detail of the locksmith's work. Good chance he screwed up connecting something or not using the right part. Or something not configured right. At the very least, you might discover the wrong power source is being used.
9V lithium batteries have a slightly higher voltage (9.6V typical) and almost 3x the mAh (1200mA). Where as a 9v NiCd only has 7.2V and 120mA.