Of course it's permitted. Saving a life supersedes the whole Torah!
Not so fast. Saving a life supersedes the whole Torah EXCEPT for the three cardinal sins [sexual immorality, murder and idolatry] and some sources maintain that Yichud is an accessory [אביזרייהו] to the first of the three aforementioned sins.
Therefore, one should try her best to avoid Yichud even in a case where her life is in danger. So her husband should accompany her in the ambulance if possible. If not possible one may be lenient on Yichud and strict on the mitzva to protect ones life and be alone with the ambulance driver. Again - if necessary one may have yichud if her life is in danger.
Many years ago there was a documented case [I read it I think in Hapardes תשכט] where a girl and a boy were on a ski lift together and when she realized that there is a problem of Yichud she JUMPED OFF and broke both her legs. This led Rabbonim to discuss if this was necessary.
My question is - Who is this tzadekes? Can she please stand up [if her leg healed]!!!! And did the boy marry her??
UPDATE: A dear reader [D.R. which is also initials dear reader] sent this: "On August 22, 1963, Ruth Friedman and Jack Katz, two counselors on their day off from camp, went on a hike at the Belleayre Mt. Ski Center in the Catskills. They took the ski lift to the top of the mountain, and hiked there most of the day. On their way back down, the ski lift stopped, leaving Jack and Ruth stranded on the ski lift. Concerned about violating the Jewish law prohibition against the seclusion of men and women (Yichud), Ruth jumped off the ski lift, breaking both her legs. (See Friedman v. State, 282 N.Y.S.2d 858 (N.Y. Ct. Cl. 1967))"
