Is it a criminal offence to instruct an employee to perjure themselves?

[DrSeuss]

A colleague has been told by her boss that she must lie while giving evidence in a forthcoming case in order for things to go the way he wants them. Clearly, should she do this, she will be commiting perjury but has he committed an offence in asking her to do this, please?

Incitement to commit a crime.

Definitely

Isn't it perverting the course of justice?

I think it takes 2 people to agree which makes it a conspiracy. Dunno though. Beyond this, I only know what I've seen in TV drama. Sounds better than what I suggested.

depends who said what to who, why they asked them to do it and what was stated would happen if your friend didnt lie. either way if your friend lies under oath then its big time trouble.

from what youve said the employer may have attempted to pervert the course of justice or

blackmail...

in England and Wales this offence is created by section 21(1) of the Theft Act 1968. Sections 21(1) and (2) of that Act provide:
(1) A person is guilty of blackmail if, with a view to gain for himself or another or with intent to cause loss to another, he makes any unwarranted demand with menaces; and for this purpose a demand with menaces is unwarranted unless the person making it does so in the belief:
(a) that he has reasonable grounds for making the demand; and
(b) that the use of the menaces is a proper means of reinforcing the demand.
(2) The nature of the act or omission demanded is immaterial, and it is also immaterial whether the menaces relate to action to be taken by the person making the demand.

menaces can be as simple as "Do what i say or you will lose your job"

your friend should go to the police immediately

I would say Perverting the Course of Justice on the limited info. Is it a criminal or civil case?

Yep, definitely an offence. He has probably already committed one alredy by asking your friend to perjur themselves. Does not matter if the evidence will be given in a criminal or civil case, it will still be a criminal offence. If your frined gives aflase evidence they will likely commit the offence of perjury. The boss is either doing an act tending to pervert the course of justice or conspiring to pervert the course of justice .... could also be witness intimidation.

Tell your friend very firmly not to do it

Any one of the following

1. pervert
2. witness intimidation
3. blackmail depending on what he is suggesting may happen if she doesn't testify the way he wants

also bear in mind that liars get found out at court. the barristers are generally cleverer than the witnesses.

What is the original case ?
HSE, Employment tribunal, crown court ?

Basically it's attempting to pervert the course of justice.