Firstly, there is no Guernsey law section on JustAnswer. I am a UK practising immigration solicitor.
Guernsey immigration rules/policy accord largely with the UK. However, there are some diffierences. The reason I asked in respect of the webpage was to establish if you were getting this from the UKBA or the Official Guernsey Government website.
You will not from the Guernsey Gov website that the eligiblity criteria is largely the same as the UKBA one but without mention of the english language test:-
In the first instance I would check with their Immigration & Nationality Division by phone to confirm the requirement with them directly, but I would expect that this is simply an ommision from the (very basic) website.
The Latvian provision that you refer to is probably as enticement by the Latvian government for property investment. There is no such scheme in the UK and I cannot find any evidence of the same for Guernsey unforunately.
There is on way of getting around the test in order to apply for settlement for the UK I'm afraid. However, the test is not as hard as I think has been represented to your fiance. I have had many clients with poor english skills and are not familar with taking tests that have passed and I should think with some basic tutoring your fiance, as a doctor, would be well positioned to pass the test.
I would take some lessons and some test papers to see how close she is presently and either take further lessons or take the test to try her luck now.
A further difficulty is that points based work visas all have english language requirements as well, and if the aim is for her to setttle permanently then the quicker route is via a spouse visa.
Sorry it could not be better news.