Talk

You need to check with the airline as some tickets allow it and others don't. She should have boarding passes for whole journey if only in Heathrow as transit but if leaving airport she will need to go through customs and immigration on leaving like a non transit person, and obviously go through security again on re entering the airport.

Can't see why it would be an issue, I assume they would grant a 6 month visitor visa on the spot (like for some other countries) when she went through immigration and customs.

www.gov.uk/transit-visa/overview is useful to read.

kansasmum, thanks for this, if she can come out I'll definitely keep this in mind to keep more time on our hands. I suspect she'll have to check in as normal for her onward flight.
KP86, thanks, I didn't know this.
AttilaTheMeerkat, many thanks for the link, it says Canadians don't need this type of visa so fingers crossed she'll be good to go!