Just curious now what type of tester like a pump type or a cylinder compression tester. The latter is most common seen first on a shelf.

If she bought a pump type like the Mighty-vac that applies vacuum and pressure she will need a universal adapter. As I remember it ran about $15.
It consists of a rubber step cone that get tightened lightly into the neck of the plastic recovery bottle.

She can then hook up a hose and apply pressure to the system while it's cold.
Just a few psi applied to cold joints, seals and hoses will usually reveal a leak somewhere.

This instrument is Very handy for vacuum leaks with hoses, bottle systems, check valves and vacuum motors.

Mighty-Vac also makes a specific coolant system tester for large metal neck radiators only, of which I have had for more years that I should tell! It is a nicely made kit and heavy duty for shops and comes with an insertion dial thermometer and reads pressure at the same time.

These are way over kill nowadays, IMO with the advent of infrared non-contact thermometers.

Phil