He always saw himself as the leader of mutantkind and he achieved that when he got Genosha, leading a mutant nation with his Acolyte worshippers. When Cyclops stepped up, Magneto stepped aside and supported him. Now Jean has come forward and wants to be a leader for mutantkind. I cant imagine Magneto will stay hidden for too long. How Magneto react to Jean? Support her like he did Scott? Or oppose her?
Jean has NEVER had any love for Magneto. She was horrified when she came back in the 80s and found out the X-men working with him as he was the headmaster to the school. Her only memories of him were of him trying to kill her as a teenager. She never as an X-men during any of his stints so she doesnt have fond thoughts about him like her friends. Their last canon appearance was in Eve of Destruction where she led a team to defeat him. (not to mention she was murdered by a man that impersonated him)
Theres also the thing about her younger self being mentored by the man that she hates. How does that make her feel?