A gentleman would stand....
If you are a girl would you let a close guy friend sit on your lap if there were no more seats left?
No
How long is this trip that you cannot stand? Seconds? Minutes? Hours?
You can either choose to make women feel empowered and sit in the seat instead of them, OR, you can embrace the dying chivalry and offer your seat and stand . . .
I believe that he should stand. First, because a gentleman would always opt to stand. Secondly, it may blurr the lines a bit from friend zone to something else. Lastly the bible admonishes men to assign women honor. Him sitting on your lap negates that.
Man up and stand.