There is a theory that if you are comfortable doing a certain distance then you should be well capable of doubling it. If she's done 20 before get some good training in at that distance, then the 46 will be a proper test but one you are well capable of doing.
Got into the 46 along with my girlfriend. Not sure if she's too happy about it yet as the furthest she's ridden before is about 20 miles. Looking at the route it doesn't look too tough though (I don't think, I'm not that familiar with south London)
https://d1ffaecguugkl4.cloudfront.net/ridelondon/live/uploads/cms_page_media/655/RideLondon_46_ParticipantsMap.pdf