Is 'Spy Phone App' safe to use?

The best way to assess this tool is to read the Terms of Service, which is publicly available here.

A relevant part is this subsection of "Installation and use of software" on the tracked users' devices:

You can only install and use this app on a device which is your
property, or for which you have written permission to install the app,
from the owner. You must make anyone who uses a device on which this
app is installed, aware of the fact that their internet and phone
activity is being monitored and archived. Failure to do the above
might be considered breaking of federal and state laws. If you install
our software onto a device which you do not own, or don’t have proper
consent to do so, we will fully cooperate with all relevant
institutions. Spy phone app is not to be used to ‘Stalk’ or ‘Harass’
anyone. Spy phone app software must not be used to monitor the
smartphone and private life of a spouse, girlfriend, boyfriend, child
over 18 without their written consent. Please read all relevant
legislation before purchasing and using this app.

Considering the possibility you are asking whether it is safe for you (as the tracker), this is the relevant subsection from the "Privacy policy":

Our app does not collect any additional info from your device than the
ones required for its successful operation. Your data is in safe
hands.

And this further down:

The data collected includes but it is not limited to phone calls, gps
logs, sms logs, system logs. We do not collect passwords or credit
card numbers.

So, the conclusion: You can only feel safe about this app if you lower your defenses and those of the "friends"/"active voluntary participants" in your scheme.

Unsolicited personal opinion - it's probably a good idea to evaluate why this app is not distributed via Google Play Store.