Stripped krank pedal hole (picture is from before)

Hi I recently acquired a second hand bike that was in “good condition” and it truly looked like it was in great shape however after a week of using it. One day when I was going up a hill I noticed a slight shake in the pedal(left side). The pedal was shaking inside the krank hole and a few minutes after it came right off and the hole was stripped. I took it to bike shop near me thinking it was just helicoil and I didn’t even have to change the krank arm. The dude said the WHOLE system had to go and I didn’t believe him and he was Just trying to overcharge. This was a New bike very new diamant bike that was 7000 kr (700 dollars~) but anyway I truly didn’t believe him. So should I just buy a new krank arm or a new krank set? Also what If I just buy another brand krank arm because I don’t think the company sells individual krank arms. And should I change the pedal? I’m just confused please help me out