The safest way, by a long shot, is to remove the SIM card while still in Canada. Then get her a local prepaid SIM overseas.
You are right, the LD add-on only works from Canada to the UK, not the other way around.
Edit: I forgot to mention that should you decide on Easy Roam after all, she can call within the UK and also back to Canada. I forgot what is costs now but I think it's $17/day or thereabouts.
Unfortunately she won't have any idea about SIM Cards when there. I want her to have her phone while waiting at the airport in Canada but before boarding if she put airplane mode on and left it on for the month would she be ok? She has done that before. I know she sometimes gets spam texts and calls and I really don't want to pay $17 a day x 30 days for junk.
Yes, airplane mode with WiFi on should be safe for traveling.
If you didn't want any roaming fee, you can also remove the international voice and data roaming add-on in self-serve. That should also disable roaming while travelling in UK
Thank you. She won't be using wifi so just airplane mode on. I will check out on self-serve. Didn't know that was an option to remove it.