Reddit reviews Jagwire Mountain Pro Brake Cable Kit, Black

It's kinda a myth that rim brakes are insufficient for ebikes.

Modern hydraulic disk brakes reduce maintenance burden, are better in wet weather, do not wear out your rim, and provide a much better 'feel' then rim brakes, but they are not going to stop you any faster in a emergency or make you safer in traffic. Provided you properly maintain rim brakes and keep them adjusted. The ultimate braking capacity is limited by tire traction and bike geometry, not the type of brakes you are using.

The biggest downside, in terms of performance, for rim brakes is when the rim is wet and cold... and if you have long descents then they can overheat the tire. Regenerative braking will alleviate much of the heating and rim wear issues.

He just needs to clean up the brakes he has and it'll be fine. This bike is never going to be a rocket sled and as long as he respects the limit of the platform and stays on top of the adjustments then it won't fail him.

The brake cables are frayed and need to be replaced. Just needs to make sure what he has works well and then it'll be fine.

Some compression-less brake cables are desirable, but not strictly necessary. They tend to be a bit thicker and bit tougher to deal with, but they do improve the feel.

Then some Kool-stop Mountain pads dual compound will help a lot with the wet weather over the cheap hard black rubber pads.

I find RJ the bike guy a very good source of information and tutorials on how to do this sort of stuff.