WhatsApp is not “social media” like Twitter or Facebook which may be received by many others, it is a one to one encrypted message. So it must have been the intended recipient who informed the airline, and in my opinion quite rightly. There is actually quite a large section of the population that have suicidal thoughts but not many have the means to do it readily available.
For a firearms licence in the U.K. your mental health is checked not just by your GP but your family as well, because you have the means commit suicide or kill others, I see no reason that the same standards should not apply to airline pilots, an employer that has any negative evidence simply must take action, it’s tough on the individual but there is life after airline pilot.