To refuse to be a witness against colleague??

You go and say what you saw. She wasn’t forced to drink and grope a man’s bollocks.
chances are she’ll get advice and realise the best she can do now is resign immediately. If the genders were reversed you (hopefully) wouldn’t be excusing her behaviour cos Shes ‘nice’.

Yes I'm sure that's what we'd all say if it was a woman who was flashed at and had her crotch grabbed too....

Disgusting attitude to take, would you be saying the same if a male had groped a female colleague?

You don’t need to make a statement about the things which “apparently” happened but you didn’t witness, only the parts you did. You just need to be factual.

If you reported being harassed or inappropriately touched by a colleague and others had seen, how would you feel if those others said that they wouldn’t support your report because perpetrator was usually lovely and you were just overreacting / needed to lighten up and laugh it off and they weren’t “snitches”?

Drinking very much encouraged

Who was doing this? If management, why?

I think if people are encouraging this, with the inevitable result, things need to be said and changed.

But yes, I’d state the facts on what I saw.

This, 100%.

Yes, you tell them what you saw. Keep it factual and to the point.

She fondled that man’s balls, that’s not on. You don’t have to say anything untrue or give your opinion on her character. Just say ‘I saw XYZ’ and leave it at that.

No one held her down and poured booze into her mouth, stop infantilising her. The company did not get her drunk, there was a free bar and she went way overboard taking advantage of it. She’s an adult.

She sexually assaulted someone, maybe a handful of people, and you think it’s okay as you like her.

That attitude is absolutely disgusting and you should be ashamed of yourself.

If it was a "lovely, usually quiet, sensible" male colleague sexually harassing women at the party, would you consider not saying anything?

Imagine you are male, asking if it would be ok to lie about witnessing a male colleague assault a female co-worker, with the justification that he is a good bloke and it's wasn't his fault anyway because the company let him drink too much.

She sexually assaulted someone and behaved like an absolute idiot
I have been to plenty of events like this and they are WORK events so you need to behave whoever you are, too many people forget that and torpedo their career
If asked you should factually recount what you saw

There was a young police officer (female) here in Wales, who has just been sacked for gross misconduct for similar behaviour at a work Christmas party.
Police officer who groped colleagues and 'twerked' at leaving do sacked | Wales Online

It seems people are expected to behave professionally despite it being a party when drink has been taken.

I would give a factual statement describing what you saw. It sounds as if there were enough witnesses regardless of whether they were sober.

Unfortunately, lovely though she might be, her behaviour was completely and totally inappropriate at a work event. You don't need to slag her off, or go into details, but I think saying what you saw is perfectly reasonable.

if other people behaved equally badly, I would hope that they are looking at investigating those too.

So... She sexually assaulted someone and you don't want to say you witnessed it?

How would you feel if the genders were reversed?

Yup,I do think you should say what you saw, and just keep it factual.

Irrespective of how drunk she was she shouldn't be touching someone else's crotch and flashing herself.