[tannk11001]

It's very likely that neither the job nor the startup will have amounted to much in 10 years, but they'll each have contributed different experiences. So it's ultimately up to your friend and whether they've got more interest in earning a years or three of startup experience, or more interest in learning in a more structured way as a recent-grad hire at the employer.

On the downside risk -- there's no blacklist. They might not see another offer from the hiring company, and if it's a small industry or city, they might have some unexpected doors closed on them. On the other hand, it's also possible that the hiring manager will be very sympathetic to a recent grad that's run into a tough situation and might even encourage him to go on his way.