You can find information about it here https://www.koodomobile.com/prepaid-plans

With prepaid, you don't have to worry about overage as you have to pay upfront everything. 

If you are looking at a prepaid plan ( you add funds in your account every 30 days), it's included 500mb plus unlimited canadawide calls and text.

If it's $30 postpaid ( you will receive a bill every month), then it's call and text only.   Everytime you access to data, you get charge $13 for 100mb. It means as soon as you used 1 mb, you need to pay and can use up to 100mb.

If she is light user, I would go for prepaid

I guess I don’t understand how it works?? If you pay as you go, and the plan is $30 a month, how do they calculate the data??

She wants to get a plan for like a month to go to BC, with about 7GBs of data but she thinks she can do this with the pay as you go. How do you figure out the pay as you go rates??

For Pay As You Go (Prepaid) plans, the $30 plan comes with 500mb (1GB if you have auto topup enabled).  To get more data you need to get a data booster ($30 for 1GB).  If you need 7GB, I would change your base plan to the $55 8GB plan.  Just make sure you make the change with 1 day remaining on your plan since changing your base plan will start a new 30 day period immediately

Thank you. So she may as well just get the 8GB plan, on a bring your own phone. I wonder if she can cancel the plan after the month then choose another when she comes back from the trip??

She doesn't have to cancel it, but she can switch plans whenever she wants. (Ideally you switch plans towards the end of your 30 days so that she doesn't lose out on days that she paid for the old plan). Plan changes can be done online or by dialing 611.

For prepaid if you don't have preauthorized top-ups and if you don't have enough funds in the account to renew your base plan your plan will be inactive. Note that after 90days of inactivity your line is automatically canceled