I haven't got BT Vision and, to be honest, I got lost with all that stuff in your post.
However one of the most common causes of not being able to change channels on any system which includes a set-top box is that the wrong source is set on the TV.
When things are working normally the TV will be set to display the signal from the 'HDMI1' source (which the BT box is plugged into), meaning that channel selection is made with the remote control for the BT box (rather than with the remote control for the telly).
However if the source on the TV set has been changed to 'DTV' (perhaps by a momentary power blip) then it will be working just as if the BT box didn't exist, meaning that it's the TV's own remote control that's needed to change channels.
So, if that's happened, your mother will be pressing buttons on the BT box's remote control but nothing will happen. The solution would be to press the 'Source' button on the TV's remote control (which is often indicated by a rounded rectangle with an arrow pointing into it), reset the source to 'HDMI1' (or whatever else it should be) and then to use the BT box's remote control to change channels as before.