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There was an article in the Guardian last week that claimed Google charged hardware companies tens of thousands of dollars to get access to Play, but apparently there are some doubts about the accuracy of the article since when I back to check on it, there was a notice that it was taken down pending investigation. Aside from that, even though the code for Android is open source, I know that Google Apps (including Google Play) are not open source and the manufacturers have to be approved by Google to get them. Otherwise, Google could not set up play to recognize the specific make and model of each device and say whether apps are compatible with it.
Your friend may be able to root the tablet and sideload GAPPS, but he or she might brick it in the process.