Send your "friend" the following extract from the Animals Act:
Animals Act 1971
3 Where a dog causes damage by killing or injuring livestock, any person who is a keeper of the dog is liable for the damage, except as otherwise provided by this Act.
5 Exceptions from liability
(4) A person is not liable under section 3 of this Act if the livestock was killed or injured on land on to which it had strayed and either the dog belonged to the occupier or its presence on the land was authorised by the occupier.
Protection of livestock against dogs
9 Killing of or injury to dogs worrying livestock.
(1) In any civil proceedings against a person (in this section referred to as the defendant) for killing or causing injury to a dog it shall be a defence to prove-
(a)that the defendant acted for the protection of any livestock and was a person entitled to act for the protection of that livestock; and
(b)that within forty-eight hours of the killing or injury notice thereof was given by the defendant to the officer in charge of a police station.
(2) For the purposes of this section a person is entitled to act for the protection of any livestock if, and only if-
(a) the livestock or the land on which it is belongs to him or to any person under whose express or implied authority he is acting; and
(b) the circumstances are not such that liability for killing or causing injury to the livestock would be excluded by section 5(4) of this Act.
(3)Subject to subsection (4) of this section, a person killing or causing injury to a dog shall be deemed for the purposes of this section to act for the protection of any livestock if, and only if, either-
(a) the dog is worrying or is about to worry the livestock and there are no other reasonable means of ending or preventing the worrying; or
(b) the dog has been worrying livestock, has not left the vicinity and is not under the control of any person and there are no practicable means of ascertaining to whom it belongs.
(4) For the purposes of this section the condition stated in either of the paragraphs of the preceding subsection shall be deemed to have been satisfied if the defendant believed that it was satisfied and had reasonable ground for that belief.
(5) For the purposes of this section-
(a) an animal belongs to any person if he owns it or has it in his possession; and
(b) land belongs to any person if he is the occupier thereof.
and tell her that farmers can and do exercise their rights. The dog could be shot and she could be sued for a lot of money.