'Cuz' (or 'cuzz') is short for 'cousin' and is used as a friendly term of address among friends — like 'mate' or 'bro'. It is common in African-American Vernacular English, US Crip-affiliated speech (where it carries specific gang significance in certain contexts) and Australian Aboriginal English. If your teen uses it among friends, it is usually just friendly. Worth knowing the gang-affiliated sense exists in US contexts. Generally harmless among British and Australian teens; ask gently if it appears alongside other concerning language.