The letter of reference should indicate how the application is actually remunerated. An hourly wage is not the same thing is as a salary. If she paid an hourly wage, then the letter should say so.
The letter can say something like "at least 33 hours per week" - and it is quite normal for hourly employees to sometimes work extra hours, is it not? Yes in call centers sometime you get 40 hours a week and sometimes less than 30 hours a week .
As for the discrepancy with the T4 - it is very small, and MORE than the minimum amount calculated, so it is quite OK. It would be a different story if the T4 said 20,000K or less, while the info in the letter suggests 23K. YES Thats right
If she wants to add an annualized amount to the letter, it can say "Ms X works a minimum of 33 hours per week (occasionally more), and is paid at the rate of 13.50 per hour, for earnings of approximately $23,166 per year."
EDIT: Or as Ozlis suggested, "a minimum of 30 hours per week" would be quite OK too. mentioning 30 hours a week wouldn't contradict what she stated in her Post ITA application ? which is 33 hours a week ?