Now the question arises on what exactly you mean. Now if you were spying on your employee on his/her personal phone and you installed mobistealth on the target phone without their knowledge, YES its ILLEGAL. Now if you had a company phone and if you install the software in the Company phone and provide it to the staff, Well thats Legal.
Its how you use the software that matters most. You are allowed to install the software on your product. So if your wife actually finds out that you had installed Mobistealth on her phone without her knowledge, chances are that you may get into trouble (if not legally, you can be assured that you are not going to have peaceful nights for the next 10 decades). But yes, ITs legal to install the software on your phone