As to the visa part of the question:
This UK govt site is a self-check for UK visas. Entering your friend’s info (citizen of Ukraine, coming to the UK on her way to somewhere else, and presuming the Stansted > Bulgaria flight departs from Stansted on the same calendar day as her arrival at Stansted and she will not leave the Stansted terminal) returns she does need a UK visa. (The web q-and-a can be misleading, however, as it does not list the particular case of an airport like Stanstead without airside transit facilities: at Stanstead, every arriving passenger from a non-UK flight will pass through UK Border Control at Stanstead.)
The International Air Transport Association (IATA) maintains a database of travel information, which is almost invariably checked by airlines before a boarding pass is issued. This is because if a passenger is refused at a transit point or destination, the airline must return the passenger to their departure point, and may be fined as well. The IATA Travel Center returns a finding that your friend will indeed need a visa unless she transits the UK at Heathrow or Gatwick (or somewhere else in the UK that isn’t Stansted; I closed the window before noting it further).
When your friend goes to the airport in Ukraine to board her flight to Stansted, before allowing her to board the flight, Ryanair will check to make sure she can enter the UK. The IATA database says she cannot, and Ryanair will not permit her to board the flight to Stanstead.
Given this uncertainty and the serious problems that will occur if things go wrong, you and your friend should consider the comment of @jcaron — with which I agree — that another itinerary would be a lot easier, a lot shorter, and probably worth the increased cost.
The safest course of action is to either a) have your friend get a UK visa before she attempts to fly this itinerary, or b) choose another itinerary that avoids the issue.
If she does take this flight, when she approaches UK Immigration at Stansted, she should be prepared to show evidence of her next flight from the UK to Bulgaria.
This answer is only partial, as it does not address the Covid-19 issues raised by this international itinerary.