No mention in the article of a mother of the child. If the mother was speaking English (or her native language) then theres nothing really wrong with the experiment, apart from the fact that the dad quit early. I see nothing wrong with a kid growing up knowing "alternative" languages as long as they have a mainstream language as well. Bringing up a child with just a fictional language would be irresponsible.
Kids grow up bilingual all the time no problem at all. My wife is German (grew up in Berlin) but as her mum was English she learnt both languages from a baby. At that stage in her life English had little use outside of the home, now her German has little daily use. She can swap between the two at will, and knowing the additional language in no way takes away from her abilities with whatever she is using as her current main language. She also learnt latin, not a language you can find many people to have a conversation with, but fantastic for understanding
For myself I like languages, foreign, historical or fictional. I like seeing the roots of the languages, how varying languages relate to one another etc etc. My own kids have English as their primary language but they all know a passable amount of German, and smatterings of other languages including I believe a few words of purely fictional languages.
A quick google confirms that this kid was growing up bilingual, his mum speaking English his dad Klingon. The kid wasnt going to suffer any more than any other bilingual kid, even if his dad had continued the experiment.
I really dont see knowing multiple languages even if some are fictional as damaging a persons life prospects.