A few wrinkles that you need to consider:

How much time is the connection?
Which airline is she flying in and out of YYZ - which terminal?
What day of the week and at what time is the US bound flight?

She need to consider the time it will take to clear US-CBP and CATSA security check. Depending on time and day of the week this can be over 2 hours.

Otherwise she just walk into the terminal as a domestic arrival, meet up and then go to US-CBP preclearence.