Fluff

Hmmm that’s what I would do personally 

Stay out of it. It is none of your business. Your friend and her partner have to sort it out.

You can't win I this situation.

What do you want to achieve? Your friend has been told. It is her life.

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I don't know what "share it" means.

Share with who? If not partner, then who, friend, the internet?

If the partner denied it you have to decide who to believe. Ultimately if you don't trust your partner you have a problem as what else do you have to base any relationship on. If they own up you have a chance to have an honest discussion. If they don't you then have to choose.

Yeah this is what I told her but he denied all knowledge. however she has it on good report from a friend that they have been seeing each other 

Ouch, that's pretty wild. The first port of call would be a serious discussion with the partner I think....