I am speculating here not knowing the full details but this is not a regular Air India BKK India UK ticket or Turkish Airlines BKK Istanbul UK where your bags are checked through and you are ust transferring onto the next connecting flight. These would be legit methods of getting back. With the destination being the UK and her being a full UK citizen UK, the rip should cause no problem. The UK won't refuse her entry even with a damaged passport.
The ticket she seems to have booked is one shown on Skyscanner or Google Flights. Its normally the "cheapest" option but for 99.9% off people should never be booked. It should be shown with that warning too! Its a bunch of unconnected low cost airlines. So indigo to India. Then possibly indigo or another Indian airline to Abu Dhabi, Shariah, Dubai etc. Then probably Pegasus to possibly the UK via Istanbul. It might not go to the UK, some of them go Budapest or Milan etc then its Wizz Air or Easyjet from there.
I believe she had one of these type of tickets. You may need a visa for India? You may need to change airport somewhere on the route. You may be allowed a piece hang luggage on one at 10kg, but 5kg on another and none the next one( a nightmare of grief and on the spot payments) If you add a suitcase to every sector this <deleted>ty set of flights becomes more expensive than a regular flight like Thai Airways one way direct to London 18000 baht ish. The online booking sites that offer these daft combination of flights should never be booked with ever. I think she had one these joke tickets and its right you can't travel anywhere on a damaged passport with exception of your own country pretty much.