How awful for her.

Assuming she has spoken to neighbours and they have just shrugged?

In which case as private legal route is blocked by cost, has she by any chance legal cover on her house insurance?

Does she want to replace like with like, fir tree?
Or take the opportunity for something else, maybe flowering tree of some sort?

Sorbus, cherry for instance

That is very unfair. perhaps the neighbours as they authorised the work have house insurance they can use that to pay for replacements.

Has she reported the incident to the police re criminal damage?

can she get in touch with the citizens advice bureau for example?

Send them the bill for a replacement plant and then take them to the Small Claims Court when they do not pay up.
May even be worth sending the Bill to the company which cut down the tree.

Chances are they are overworked, but aren't there charitable legal organizations for retired people in the UK?

I know just how your friend feels, Cam, because my neighbour came onto our front lawn right up to our house wall and cut the medium height laurel down by half even though most of it was on our side. Like your friend we don’t have a demarcation line between the front lawns and all the neighbour is entitled to do is cut off any branches encroaching on his side. I was furious with my neighbour and told him that what he’d done was against the law. He laughed and said to his wife, “There, see what you made me do”. I didn’t find it funny at all.

If I was your friend, Cam, I’d just get a replacement because resorting to the law will cause animosity that she will have to live with.

Thank you all. She hasn't had the courage to confront the man (who's a bit odd) so I'll go round and have a chat with her later. The people who cut it down deny all responsibility because the man said it was on his land.