poor/slow internet at weekends
 

My friend in Sydney's Blue Mountains has a good internet connection usually - by ethernet to his desktop tower; but at weekends his connection is poor or missing. However his partner is there at weekends using WiFi to a laptop from the same modem, and has good internet at weekends. Would WiFi usage get precedence over ethernet from a modem ?

Not normally, no. Is this when both of them use the Internet at once?

Could be a DHCP issue with the router, only allowing one connection at a time, maybe?
Etherley is usually more reliable than wireless so the situation you describe is very odd.

I am not there when it happens - all I know is that the ethernet gets poor 'service' but there is no complaint about the WiFi - which is simultaneous some or most of the time. Like you, I expected ethernet to be more reliable than WiFi, and I also know that WiFi degrades as the number of users increases. I have advised to move the ethernet cable between the four modem ports, and that seemed to provide some improvement at times. The modem was new in November last year.

Sure the partner is not inadvertently tethered to the laptop using his/ her mobile phone data?

loss of etherenet access during weekends
 

The partner that is only there during ethernet-trouble times has and uses a mobile phone extensively. I think the tethering is a non-event, because when I rebooted the modem in a desperate attempt to get improvement, the laptop's WiFi was interrupted.