GERD and oral health implications. Teenager who has done root canal, has lost two teeth as well as she has 7 fillings and 6 cavities. Should we be concerned as well as he recently had a cleaning done and the dentist said the gums were infected?

GERD and caries. Yes, GERD does have the potential to make dental caries worse. It doesn't have to though. Through a combination of anti-reflux measures and medication to keep the reflux to a minimum, she can lead life without any affect on her teeth whatsoever. Work with her doctor to get her GERD under good control! I hope this helps.
Be concerned. GERD must be brought under control (see a Gastroenterologist). He's oral hygiene may require modification. May need more frequent professional cleanings. May need diet adjustment. Talk to your Dentist. Ask if referral to a Periodontist is warented.