In the tutorial video, it specifies running in Wayland but I'm in X11.
Also from Googling, Arch Wiki:
Waydroid only works in a Wayland session manager
I know nothing about switching from X11 to Wayland, what that would involve or how that would affect everything else on this system. (But I suspect it would be major.)
I see a few comments about something called "Weston" to allow a "nested Wayland session" within X11. Again, I know nothing about making that work and whether I could realistically expect my mum to do it whenever she needs to use WhatsApp.
As for alternatives to WhatsApp, it would need to be something that wouldn't get a Chinese person in Hong Kong (and who often travels back to mainland China) into trouble just for having on their phone. Things like WeChat, Skype and Zoom are acceptable to the mainland authorities, WhatsApp is excusable for someone working In HK, but things like Signal are out of the question. Just having it on their phone could make that person a target for investigation/punishment.