What would I do?
Tell her she's an idiot and that it's a terrible idea during the initial conversation and then watch it play out like a car crash in slow motion because she'll do it anyway.
I'm not taking on my friend's stupid dramas anymore. If they want a life like a Jerry Springer episode, that's their choice. I wouldn't entertain her drama when it all comes crashing down. I'd distance myself from it and probably her if our values were miles apart. I don't agree on deceiving people into parenthood.
I wouldn't tell him though. If it happens, he is silly leaving 100% of the contraception responsibility in the hands of a chick he's known for 4 weeks too. I wouldn't get in the middle if it has the potential for you to get stressed out, victimised and attacked for your efforts.