If your former colleague or former employer wants you to attend, and you don’t want to, you don’t have to unless the tribunal orders you to attend. The tribunal is very unlikely to make an order unless one of the parties asks them to. If they do make an order, it doesn’t mean you have to give the party that has asked for the order any assistance in preparing their case: you don’t have to talk to them beforehand if you don’t want to, or give them a statement of your evidence.
On the whole, parties tend not to ask the tribunal for witness orders unless the witness is basically willing to attend anyway. That’s because a reluctant witness normally won’t have provided a witness statement – so no-one knows for sure what they are going to say until they say it to the tribunal. That means calling them is risky. (There’s a bit more about this here.)
The situation is a bit different if it’s your current employer who wants you to give evidence. They can’t actually force you to do so – but they can make your life miserable if you refuse, and they might even discipline or dismiss you. So you might find yourself in a position where you’re under a lot of pressure to give evidence.
If your employer wants you to say something that isn’t true, you should stand your ground anyway: telling lies to the tribunal under oath is perjury, which is a serious criminal offence. If you’re dismissed for refusing to do this, you should certainly complain of unfair dismissal; in some circumstances, you might also be able to complain of discrimination by way of victimisation.
If your employer wants you to tell the truth, but you’d just rather stay out of it, you will have to make a judgement about what they’re likely to do if you refuse, and how strongly you feel about not getting involved. But bear in mind that the more closely you were involved in the events giving rise to the claim they want you to give evidence about, the more likely they are to be able to justify disciplining you for refusing to obey a lawful management instruction.