Dump her, dude. If she is flirting with someone else, theres plenty more fish in the sea. Besides, she has a son who is 10 years old. If the kid was an infant, it would have been ok to be a surrogate father as he would not know that you were not his real father. But he is 10 years old, common, he might wanna start a fight with you. So again, find someone more suitable for yourself.