Unlike you, I’m not a lawyer but here’s my two cents.
The legal definition is a little murky depending on what jurisdiction you’re in. Many places distinguish between being an accessory and being an accomplice, with accessories having less complicity. But just having foreknowledge that Bill intended to commit a crime (he told Dan he was going to beat somebody up, which is assault) could be enough to qualify Dan as an accessory.
At that point it becomes an issue of how much complicity is necessary to trigger a charge of felony murder. Again it depends on jurisdictions but I’m guessing some jurisdictions might say that being an accessory is sufficient participation to justify the charge.