[JamesVI]

Like LinuxBender said, document everything.

You don't say where this happened; local laws and customs will prevail. In California (for example) most companies will require you to report sexual harassment (which is what you are describing). If you (or coworkers) see it and say nothing you can be fired for failing to report when it eventually gets out (which it will).

Write down everything you know personally and everything that you have been told. Take it all immediately to HR. Don't ask your colleague what they want to do, and don't take this to the manager's boss.

HR will talk to your colleague. If they say nothing happened, or that they welcomed the attention, then either the manager or the employee will be reassigned so there isn't any real or perceived power imbalance and they can continue to do whatever they both want. If your colleague says that the manager got them drunk, or made unwanted advances then HR will follow the necessary process (involving the manager's manager) to terminate the manager.

I've managed teams in California, and I'm currently in a long-term relationship with a former co-worker, so I've navigated these waters from both sides.

[anydden]

Should I talk to the colleague ?

[JamesVI]

At this point I would say no,

Document and hand off to HR. If the colleague comes to you and asks if you reported to HR then you can be honest and say you did because you were concerned for their well-being.

Talking to them doesn't really help them. It forces them to decide if they want to endorse you talking to HR or to claim it was "no big deal". HR is (supposed to be) trained to deal with this kind of situation and should be better equipped to decide how to proceed.

[anydden]

But that would be too much right ? I don’t have any proof that it was harassment , could be just normal drinks I also offered her one , but this guy was certainly trying to touch her shoulder hold her hand , and call out her name multiple times

[JamesVI]

The situation you witnessed and heard about made you sufficiently uncomfortable to create a throw-away account to ask HN for advice. That level of discomfort is sufficient to report it to HR. For all you know there have been other complaints made in the past and this incident will complete a picture of a pattern of troubling behavior.

It isn't your responsibility to gather proof or to make a career-impacting decision about these two people, that's for HR, legal and senior management to deal with, and they have the tools and processes to actual investigate. (Check your employee handbook, companies I have worked for in the past require employees to cooperate with HR investigations and refusing to comply can lead to termination). If there really wasn't any problem then HR will take no action (they don't want to get sued for wrongly terminating a manager either).