She can stay in the Schengen area of which France is part for 90 days in every successive 180 day period. She can return to Canada and apply for a standard visitor visa however her chances of getting it in the immediate term are slim. She failed her landing interview, likely for the reason below. I had a similar six hour ordeal interview with UK immigration but ultimately passed and was let in. The secret to passing landing interviews is to be adequately prepared, just like you would be for a visa interview with supporting documents etc.
Immigration Rules Appendix V: visitor rules
Genuine intention to visit
V 4.2 The applicant must satisfy the decision maker that they are a
genuine visitor. This means that the applicant: (a) will leave the UK
at the end of their visit; and (b) will not live in the UK for
extended periods through frequent or successive visits, or make the UK
their main home;
I similarly use the UK as my base for European travel and planned an entry through Calais this past May which I ultimately changed and came to the UK direct. The best advice will be return to Canada or redo her travel plans to be feasible without having a base. Of course she can attempt to enter again tomorrow without a visa via the same route or by air via another route or sea, nothing precludes her from doing so. Nevertheless the chances are slim and she can be banned from entering the UK after another or subsequent attempts so she should be careful.
You do not want to get into a tail spin of successive refusals.