He is now able to install the app. Two things happened: He cleared the cache of the Google Play app as you suggested and I put out a smaller .apk just to try that as well (I used the "generate one apk per ABI" Android option) . The previous install bundle was approx 33 Mb, the new install is down to 13 Mb.
So all is good now, although I am not fully sure which change made the difference.
Thanks
JB