personally, I dont think your hypothetical hits the mark regarding a collective moral obligation, in that the decision to request the help of others/another in itself, does not require any assistance....
However, here is a twist, if you had a gun or another sure fire way to force others to assist you, would you morally justify this use of force to save the fat man?
by extension of this thought process, i would ask to those more religious on the forum, isn't God pro-choice?....i mean, free will and all that? isn't the decision to be "pro-choice" or pro-life" a personal one, between you and your god to judge you when the time comes? sure, you can express your opinion, you can educate others on your opinion, you can warn others about how god will punish them, but to force "god's law" on man sounds like taliban stuff to me.....