As to the visa part of the question: information available online is in conflict as to whether a visa is necessary or not.
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 Stanstead > Bulgaria flight departs from Stanstead on the same calendar day as her arrival at Stanstead and she will not leave the Stanstead terminal, she does not need a UK visa.
On the other hand, 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 Stanstead; I closed the window before noting it further).
When your friend goes to the airport in Ukraine to board her flight to Stanstead, 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 it is either very likely or certain that Ryanair will not permit her to board the flight.
I have no resolution to offer here. One of the online sources is incorrect, but it is unclear from outside the systems (we’re just interested amateurs here on Stack Exchange) which is in error. In the past, both systems have shown inaccuracies online.
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.
If she does take this flight, when she approaches UK Immigration at Stanstead, 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.