Nobody broke the law lol.

I believe they have like a month to comply.

The just asked for a ticket in the app, to make their lifes easier. If OP doesn’t want to, they still have to comply though.

Now I remember why I hate working directly with customers.

They were very friendly imo. No need to speak legalese or to be rude.

Just tell them that you can’t or don’t want to install the app.

If they don’t help you, then you proceed to remind them that you are not required to install anything for them to comply with GDPR.