I’ve been using DLAN for over a decade, Devolo for around 4 years. They have been 100% reliable.
I spend around 8 - 10 hours a day connected to L2TP VPNs with no dropouts.
It probably isn’t the Devolos per se. It could be something on the line that is causing problems. I did have a 6 plug extension with a transformer for charging up to 4 USB devices. It was very “dirty” and caused enough interference that the throughput of my DLAN dropped to around 5mbps. Removing the dirty extension lead from the set-up restored it to full speed.
Does he have a fridge, for example, on the same circuit? They can be very “dirty”, when they turn the compressor on and off, causing huge spikes. Likewise microwaves can cause a lot of problems. The route between the two devices needs to be as free from noise makers as possible.
We also have a couple of users who have dodgy DSL connections, the VPN drops repeatedly, because they can’t get a decent connection between the house and the cabinet, especially when the neighbours are at home as well.
Another thing to try, which has often had success for our users who suddenly have problems, is to go into the Networt Setting (new Windows 10 settings) → Status and scroll down. Towards the bottom is an option to reset the network connection. This usually works on setups that refuse to connect at all, rather than random drop outs, but is probably worth doing, just to remove the PC itself from the equation.
(my setup is DSL router in the hall → DLAN in hall → DLAN in office → Firewall → switch → PC / WLAN access point → wireless bridge to other access points. This works 100% reliably, but the DLAN can have problems with dirty devices on the line, as described above.)