So: yes - you can switch it on/off directly with the button (delay approx. 1 second). Unfortunately, the app does not send any messages when it is switched on/off. However, you can control the Water Controller externally via the Internet using Blooth Bridge (we have already done this from abroad :-) ). Apparently it's also possible to connect Hombli to Alexa and/or Google - but I haven't done that yet. Maybe you could "set" something like a notification. In any case, it would be a good thing if you could control a message! Any settings are made via the app (in my case the one from hombli), but there are also third-party apps that partly support such valves (e.g. Eleison), maybe you could activate push messages there?
Otherwise, simply set up a watering schedule for your father, then he won't have to worry about it at all. Connected to the rain sensor, this can even be interrupted automatically when it rains.

SOOOORRRYYY!!! Must correct myself....

I took a closer look at the Hombli app and it works! But you have to create a 'scene', you can switch on messages for different actions - I just tried it myself and it works! :-)