Best friend to dating
They're not hiding the fact their seeing each other. It really doesn't matter if the woman he's dating is younger/older or attractive or not.
At this point you don't know whether this is true, or gossip with no real foundation. Since it's your best friend then i think you should tell her. The woman is free to date him, as he is seperated and free to date. Are you suggesting she shouldn't date him because she knows his wife?
I also can't help but wonder if maybe this has been going on for a while, perhaps even before the split? Although as factually as possible: they were seen out together, definitely a romantic date rather than colleagues having a drink.
The gossip will get out there in the office soon and people will start talking about it, if they haven't already.
They're both adults & are fully aware of what they're getting involved in.