How did my friends stalker find my mom's phone number?

My friend has been dealing with a particularly disturbing stalker who shows up at her house at night and leaves her creepy messages on her ring camera and follows her and since she has him blocked he messages and calls her mom and says all sorts of disturbing things.
I made the mistake of confronting this guy on fb to let him know to back off. It became pretty apparent how unhinged this dude is and I blocked him. Now …

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