Too young girlfriends sometimes look too odd

I am still speculating over a situation that happened last weekend, of how things might get odd sometimes depending on the situation.
We were invited to a birthday party of a kid of our friends. There were not many guests invited just close family & friends. So it happened that mother of our friend came to visit them from overseas and that his father who divorced his mother long ago was also there. Then the second father's wife who our friends stay with very close and who helps them a lot was also there. It would be all okay if that was all fathers former women at that party, because the two sort of found common ground for conversation (they are of about same age) and maybe because neither of them felt disadvantaged in front of the other ( he divorced two of them).
But no... he did not come alone, he took his new girlfriend with him, who is somewhat 20ish, somewhat 30-40 years younger!
She is asian, that made me thinking that I might be wrong about the age, but as she is a uni student means I should be about right.
She looked and felt very lonely in that company, because after realising how weird he looked, the 'father' left her on her own too.
I tried to keep her company, because honestly I hate when someone is being left out like that, so we spent almost all evening together...
Talking with her was not bad at all, she's smart, very determined, and what is the most important she is absolutely honest with the 'father' about her feelings towards him - which is pure business relationship - living with him while studying in uni ( and him paying for it).
I understand that is not really someone else's business, and that we should not judge the age difference as we cannot know 100% what is happening there etc... but when people themselves make you a part of it, it's really hard not to.
As to me the only thing of how to make it all better was not to bring his new girlfriend to the party...
What are your thoughts?