What's with all the complicated and expensive alternatives? If her current boiler was heating the house well enough before it broke, replace it with similar. Don't listen to all the shite about the government's ambitions, they don't even kick in until the mid 2030's, by which time her new boiler will be knackered or.......well, she is a pensioner.
Baxi bermuda is a back boiler that sits behind a gas fire ,but is independent from the fire (they share the gas supply and flue) the fire does not heat water .
this style of gas back boiler are no longer made i don't think
that said, parts are available and maybe everyone who has looked at it would rather fit an expensive new boiler than replace a part and only be billing an hour of labour .
I'll generalise a bit here but the case in question may well be this ...
as it's a back boiler there will be an indirect tank ,valves, pump and controls in an airing cupboard
the ideal place to fit a combi (subject to being able to route a flue out )
BUT ....
whist most airing cupboards tend to have flow and return pipework for heating circuits ,a cold incoming supply and domestic hot water pipes to the taps ,all stuff you need to fit a combi
they don't have gas pipes
electric heating and air source heat pumps etc are very expensive ,even with a grant
a good local independent heating engineer may be able to fit a decent combi cheaper than going down any grant route (looking the grant route requires £2600 investment minimum, might be much less if on benefits )
cap/disconnect the flow and return to the back boiler location, at the airing cupboard
decommission the baxi back boiler, run a new gas line to the airing cupboard, and fit a combi and flue
as the heating pipes, rads etc are probably very old, a powerflush would be good practice
alternative to just a combi is a
storage combi clicky which is a fantastic bit of kit for the space it occupies