App uploaded and approved to iTunes, yet users see old app launched after upgrade

We have made an update to iTunes for our app.

The point is the previous version, say v1.0 was developed by another developer and we developed this new version from scratch with not even a single line of code reused for developing v2.0. There are drastic UI and functional changes too in v2.0

Now, the v2.0 is approved by iTunes and is live ready to be downloaded. However, my client who had the previous version, v1.0 installed on his device is complaining that after downloading v2.0 the app works correctly and after sometimes, older v1.0 starts running.

Can anyone point us to a right direction, since with no possible linkage between v1.0 and v2.0 apart from the Bundle ID how could such issue appear.

Just to confirm, that client is not bluffing we have seen this issue appearing on his device.