'ken oath. There are limits to fence sitting.
Ripley even says to Lambert (though it was ultimately cut) that the reason she wanted to proceed with Dallas' plan rather than abandoning the ship was the chance that Dallas could be alive. If there was a chance to save Lambert - Ripley would've done it.
That still relies on her knowing
Lambert was dead, instead of presuming.
All we know was that the woman was bleeding and apparently held up on something. The human body can withstand amazing levels of mutilation before it physically gives out.
Ripley saw what she thought
must be a dead Lambert.
She did not necessarily see
one, even if the chances of it might have been high.
As ever, there was nothing 100% about the scene. We see Ripley's reaction being to simply recoil. She assumes
Lambert is dead. She might even be right. There's just no proof of it on screen.
Heck, let's say Lambert had half her intestines pulled out of her vagina, was missing a lower jaw and had all the skin from her face peeled away. That's no guarantee of death, but I'm sure if that's what Ripley saw, she would have thought it was - even if Lambert might be in shock and comatose.
I know I wouldn't be checking for a pulse on something as messed up as that.
It's all about wiggle room. Was there enough for Lambert to potentially be gestating the same thing inside as Dallas was? Yeah. Is it likely? Probably not. But 'probably' doesn't matter when it comes to crafting future stories.
And if we take Brett into consideration, Lambert need not have even needed to survive the process.