She will run a serious risk of having her visa cancelled if she tries to stay 6 month and then return almost immediately but equally the 6 month in 12 is only a guideline. My wife had consecutive long visit visas over a 8 year period and just before we settled in the UK we made several visits ranging from 2 weeks to a few months and we were grilled on one trip and told she should apply to settle if we planned to remain in the UK. I explained why the multiple trips (Sick mother) and she was allowed in no worries. We then decided to move back and she obtained a settlement visa and we moved.
NB at the time there were no exit checks so she could easily have been in the UK for 6 months on each visit