Apply for a fiance visa from outside the UK, but not country of citizenship

Hi - my partner and I are in the early stages of preparing to apply for a UK fiancé visa, and then convert to a FLR.

My partner is a US citizen, and although she could be in the US when she applies, she may well have a temporary work opportunity in Europe at the point we will be applying. Will she be able to apply from an EU country that is not her country of citizenship / permanent residency, or is it better for her to return to the US for the durations of the process?

I imagine that the answer will be something along the lines of 'it depends if she gets a temporary residence permit / visa in the European country in question' but thought I'd ask just in case!

Thanks in advance.

She can apply from any country where she has legal residence and the relevant visa, ie is not in the EU country as a visitor (business or tourist).