If there was someone else to help what is the problem? Neither lifeguard not a physician has the legal duty to endanger his own health.
Moral (not legal one) duty to take a risk would come into play if there was not other person willing to do it.
But there is an issue why the "mouthpiece." was not there?
Only one thing in the whole story is for certain:
An ambulance-chaser will sue the government (local, county, state, or federal, whichever can be named) for multi-millions of dollars, and the taxpayer will pay the bill....the lifeguard, of course, will not pay anything at all....except to live with the consequences of his (in)action.