i would go for PACELC. which is based on CAP but describes a better behaviour.
And CAP is only relevant for distributed systems if he just has 1 server with 1 core in theory he wouldn't have to concern himself with it :)
If he's going to run a single-server (in other words no partition), then it's easy, go for something that's ACID compliant and relational, unless he plans to build massive graphs in which case Neo4J might be a good choice.
ofc! In my experience devs rarely know why the select a database they only know it's hip and they wanna use it :D .... Did it myself till i started reading about the differences between the engines, martin fowler goto talk about nosql and other things like the jepsen talks :D ... i love jepsen Gg ...