You can only cancel an Extension of Stay, not a visa. If you have an Extension based on work, you cancel your WP then the Extension and leave the same day, or get the 7 days to leave mentioned by
She will need a Visitor visa if she's just visiting. She applies via VFS in Bangkok or Chiang Mai. If she plans to live in the UK she needs a Settlement Visa. Easy enough, if you meet the financial requirements and she passes the English language test.
There won't be any repercussions during the visit, but the next time they apply UKVI might notice that they stayed longer than they said they would and this might affect their decision about a future visa. If there's a 'good' reason that they stayed longer than they applied for, it may be ok.