If you can talk him into accepting a cable, there are options to extend ethernet :

https://store.ui.com/us/en/pro/products/long-range-indoor-outdoor-ethernet-repeater

When cat5/6 cable is described as ‘solid core’ it does not refer to the plastic divider. Rather, it means that the wire is solid copper, as compared to CCA ( copper coated aluminum).

If you have a lot of terminations in your future, I would recommend using pass-thru ends and crimper. Much easier to get it correct.