my bfs stupid comments

im a teacher and today i ran into a teenage student at the grocery store with her mother in tow. I quickly greeted her and introduced her to my bf who doesnt hesitate for a moment to greet her mother with "oh who is this? is this your sister?" i looked at him incredulously. first of all it sounded like a corny pickup line and secondly it's completely ingenuine cause my student is 17 and the lady looked like she was 50 going on 70 and not fooling anyone about it . (PS my bf is the same age) i told him after to stop using that (stupid) line because he uses it all the time in similar contexts. Does anyone agree with how i see this comment?