It must be immediately preceding and you must be 'currently resident'. She cannot file a California divorce from the UK, she must physically be here. But establishing residence does have some loopholes. The exceptions are something like a vacation, a job secondment, or a military tour. The issue is whether or not you have 'established residency' in another jurisdiction.
So let's say I do the proper paperwork with UK authorities and move to the UK, get a flat, open utility accounts, move my finances to a UK bank, but still keep my US home. For all intents and purposes (and assuming no evidence to the contrary) I have established UK residency for CA divorce purposes. If I decide I don't like it and hop back here in say, 3 months, my clock starts over again barring a ruling from a judge. But if I do all this and let it be widely known that it's only 'for a little while to tour the country and wave from a few balconies get a taste of British Life' then I go home 5 months later it could be determined that I never established residency in the UK as I always intended to return and I would meet the 6 month requirement right away.
At this point only having been gone about a month and not having bought or rented a residence, opened utilities, etc, she could certainly argue that it was a 'temporary absence' but the publicity around it being a move of long duration and especially having enrolled the hamlets in UK schools as well as saying they were here for the duration of the educations (so what, at least 10 years?) would seem to belie that. I doubt very much that a judge would consider a prospective and 'avowed' 10 year absence as 'temporary'. But her announcement that she would be back and forth from here to there could mean that she never actually loses her residency. The time split isn't set in stone, a judge would have to rule but I feel safe in saying that she'd need to spend more time here than there.
There are also residency exceptions if your spouse has been at least 6 months in California even if you've moved away. But that wouldn't apply to MM as H isn't here. It might apply to H (who established CA residency some time back) if MM moves back and he wants to divorce her under CA law, which he might if there are inherited funds involved. If he moves back in that situation he wouldn't need to wait the 6 months.