An important note: If the player is choosing to lose, it's not a take-out, it's a concession.
In this case, the player is saying "I don't think this fight is important enough to be worth spending consequences on; can I avoid that by just losing now?" And negotiations continue from there. Admittedly, the player is negotiating from a fairly bad starting point... but it's still a negotiation, because if you started with "Okay, so you wake up in a hospital two days later missing your left eye," the player could still go "Woah, no deal, I'll take those consequences after all."
And in this case, killing the PC should be entirely off the table - even if the biker was going for a kill, the terms of the concession would have to include something like "But the cops show up before he can finish you off; you're battered, bruised, and have acquired a level of police attention that's going to cause problems for you if you have to do anything at all suspicious in the next few weeks, but hey, you're not dead - and from your character's point of view, that's a good deal."
In this case, I'd probably insist that the terms of the concession include a minor physical consequence, and some significant loss that will impact more than just this game session; it doesn't need to be phrased as a moderate consequence, but a temporary aspect like "Important Witness In A Police Case" could be highly inconvenient, for just one example.