Read the following UKBA link sections 41 & 44 (ii) to see if this answers it for you
http://www.ukba.homeoffice.gov.uk/polic ... les/part2/That said, sometimes at the airport entry point, the immigration/passport control officer may simply restamp another six months in that passport, or consider the first six month stamp as it being valid till the final date (time period) in which it was issued & valid till.
In other words ... six months max from the date of the issuance of the initial entry (stamp in passport) in the first place, even though someone may have left for a brief period to visit somewhere else in the EU ( & had not returned back to Canada) then, re-entered the
UKFor example, a commonwealth citizen entering as a general visitor, stays 4 months then goes off to somewhere else for a month then returns to the UK.
Its possible (but not always the case) to get re-stamped another 6-months or for an a shorter duration up to and including at first 6 months or a wee bit longer.
Let us know what happens with the exit & re-entry issue & what she decides to do
Disclaimer: I am not an immigration expert & anything that I post on visa & immigration is based soley on more than 30 years of personal experience & interaction with the UKBA & Home office.