Yes it is not legal to use on the phone that you do not own.
Is It Illegal To Purchase Cell Phone Spy Software To Hack Into Spouses Cell Phone?
No, spying is technically illegal unless it is for government purposes meaning that if your wife is not a terrorist and you don't think she is planning on killing anyone than maybe you should use the new found method of asking her what you want to know. Relationships are kind of meant to be built on trust.