EEA2 and new (non-EU) citizenship

[obormot]

How UK immigration treats cases of acquiring a new citizenship (non EU) after one gets residence papers to live in UK?

My mother, originally Russian citizen only, got EEA2 5-years visa as my dependent. It is on a separate piece of paper, not in the passport. After getting EEA2 she changed her Russian passport to a new one (old one expired), but this seems not to be a problem, no-one had any issue with that when she recently traveled.
But she also recently (after getting her EEA 2) acquired another citizenship (non-EU, specifically, USA). Now we plan to travel to continental Europe and she will need to show her US passport in order to go there without a visa (Russian citizens need Schengen visa).
Could she have a problem if she shows at the UK board USA passport and EEA2 paper (which was issued based on her Russian passport)? Or should she only show her Russian documents at UK board?
Does she need to inform UK immigration authorities about her new citizenship?
Thanks!

[Jambo]

At the border she should show her Russian passport and RC. At all other checks (airline, exit control) she can show the US one.