...being aware of the policy? my neighbor ask me this

She would need to prove that she has an insurable interest, i.e., she would be financially harmed if he died. It's not going to happen unless they are doing business together, which they probably aren't.

Without his consent, it is illegal.

People here have given you a lot of correct information and some incorrect. As for insurable interest, if there are children involved, there could be insurable interest. The part about "without him knowing" is what would bother me. Some companies do not require physicals or even saliva test until you reach $250,000 or more, so she could take a smaller policy without him having to take a physical or even saliva test. Can she is the question, and the answer is she could forge his name and do it if she finds the right agent and the right insurance company. But be aware that fraud is a legal problem and one I would not want to take.