My friends long term girl friend has indefinite leave to remain in the UK she has lived there for around 9 years but now has been in Thailand for 20 months does she have to return back to the UK within 2 years to keep her visa ?

Yes, ILR will lapse if the holder remains outside the UK for a continuous period of 24 months or more.

Once it has lapsed she will need the appropriate visa to enter the UK again.

This could be a returning residents visa; but as it says there

If you’ve been away for more than 2 years

You won’t get a Returning Resident visa unless you’re also able to show what exceptional personal circumstances have led to you being out of the country for this long.

You must provide enough evidence to show:
your strong family ties to the UK
you lived in the UK most of your life
your current circumstances and why you’ve lived outside the UK

There are exceptions to this, for example her ILR wont have lapsed if her spouse/partner is a member of the UK's armed forces or diplomatic service and she has been out of the UK for more than 2 years because accompanied them on an overseas posting.

If she does return within the 24 months and if UK immigration have reason to believe she actually lives outside the UK and is seeking entry to the UK not to resume residence but for a visit or simply to prevent her ILR from lapsing then they could cancel her ILR on the spot. She would be allowed in as a visitor on that occasion, but would need the appropriate visa to enter the UK in future.

Of course, once she had ILR she could have applied for British citizenship once time qualified; three years residence if the spouse or civil partner of a British citizen, 5 years if not. Had she done so, she could, of course, stay out of the UK for as long as she wished and always be able to return at any time; just like any other British citizen.

As both the UK and Thailand allow dual nationality, doing so would not have effected her rights as a Thai citizen in any way.