"~ㄴ 줄 알았어" can mean: 1. I thought 2. I knew It depends on the context. Without seeing the picture, it's hard for me to know what exactly your coworker wanted to say, but "모간인 줄 알았어여" might be translated as : 1. I thought it was you or 2. I knew it was you

Using 여 instead of 요 as an ending particle is called '-여'체. it is used for texting and has childish nuance. 모간인 줄 알았어요. means "I though that it was Morgane" 도와를 줘서 고마워요 => 도와줘서 고마워요 or 도움을 줘서 고마워요