He may have told her he was in love, but his hormones just wanted him to be 16 years old again.
I think it's prefectly normal for a 53-year old man to suddenly find himself infatuated with a pretty young girl, although 16 does cross the line.
But acting on those feelings puts it in the category of premeditated stupidity. He chose to make a perfect fool of himself.
I do not think it is right for his employer to fire him, however. Even if the email did come from his work PC.
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Diligence is the mother of good luck.
Ben Franklin (1706-1790)