It really depends on how big the first floor is, how many windows, how many doors, is there a basement with potential break-in possibilities, is there access to the 2nd floor by ordinary means? An alarm will only do one thing, it will tell you that someone is in your house and notify the police if you pay for that feature. For real safety you want proximity sensors as well as well placed security cameras. You will also want security film on all glass windows and glass doors, steel reinforced door frame for any external entry points, this will buy you time to prepare or notify the police. Is he preparing for a regular burglary or a home invasion? Once he figures that out he can move forward with how he should proceed.