It might be this
Not for sure. Because ads are not showing even if the app is in Google Play.
That is probably because he has too many accounts and they have stopped serving them.
First problem to solve is too many accounts, then he can sort out the problem of the ads not showing as I am certain that the two are connected.
Once he has One account he needs to Reinstate that account and only server adverts from that account.
I am sure that once he has reinstated the one account within 24 hrs the ads will be showing again.
I guess we’re not yet gathered enough knowledge about Lack of Ad inventory. It was last August and I have seen a thousand of posts about this problem. But, unfortunately we’ve no Solution for this… We can only guess.
This is the best I have found so far but you have probably seen it already
Already seen but does not make any sense. 
I am sure that if All other accounts are deleted and you have One account active and the right ads placed in the right place (especially custom-sized ads) then the adverts should start showing again in 24 - 48 hrs depending on location.
Try it with test mode and if you get a test advert (banner) showing in the companion then you advert code is correct.
It is most likely that the account has stopped showing ads because there are too many connected accouts serving adverts to the same location.
If you only have one account check your custom adverts and their size.
Test banner ad in test mode with Companion.
" If you have just created your Admob account, it may take some times and some requests to deliver the first ads. I saw somewhere that people had this error too by using custom size for banner, so be sure to check :
Sometimes it takes a while for the system to recognize that it is serving an advert to a genuine app.
from what I can work out there are a few ways you can cause this issue.
- Have too many accounts, they will stop serving adverts.
- Not waiting long enough after first opening the AdMob account.
- Custom sized adverts too big or too small for their placements.