Wi-Fi for my neighbors - does that have any disadvantages for me?

My neighbor does not have a landline, just a cell phone with top-up credit. Now she takes part in video conferences and she also has a large circle of friends with whom she exchanges Whatsapp messages. I have already set up her tablet via my guest account, now she would like to have her mobile phone integrated into my guest account because of WhatsApp.

Does that have any disadvantages for me in terms of performance / bandwidth? I myself am permanently logged into my home network with Alexa, a smartphone and my laptop. I also stream videos and music from time to time.

Of course, you then share your bandwidth with her.
Depending on how "wide" it is, that has an impact on the part that is left for you.
If she uploads larger data via What's App, that can use up your entire bandwidth for the duration of the upload.

So it depends on what exactly she's doing.

Nope, I'll do that with my neighbor too.

Whatsapp messages alone shouldn't cause problems.

But you can't just send text messages via What's App.

You share bandwidth (if you have a normal connection).

You're only liable to him if she does something illegal.

, and how should he know that she doesn't do anything illegal?

Right. But she won't send a 500mb video. Video calls are sure to go up a bit… But I doubt that he would really notice (depending on how big his bandwidth is)

Not more than before… You actually have to cut constantly when you are no longer there, because otherwise illegal content would be subject to recourse.