If not, and the options are unvested, then it is just a job - you didn't receive anything the day you came onboard that someone off the street could not have negotiated.
Unfortunately, your cofounder is no longer a cofounder, but a coworker. The reason for advance notice would be the same as in any other workplace (i.e. a personal relationship) as would be the notification expectations to your new employer.
The unvested options exist only to place a value on your leaving. If they are not worth as much to you as leaving, then there's nothing wrong with going - and if people are going to be pissed, they'll be pissed anyway. The sort of people you want to work for are those who value your longterm happiness more than your warm body filling a production schedule.
Good luck.
[edit: I'll add that if you didn't cash out somewhat, that was a really dumb strategy by the acquiring company because it did not give you any immediate benefit for having ground it out all those years]