Does the Digiface still work? Then you don't need to check as the fault was wrong or missing grounding and not a voltage/current initiated from the Digiface itself.

In this case I would expect the headphone output of the Digiface being damaged as well.

> but the buyer informs me that the host  red light goes on after a few minutes usage

That usually happens when there is not enough power. I would try a different power supply first. Of course I can not rule out that the DF got damaged in a way that this kind of error shows up.

From a technical point the problem that occured is fully clear and nothing  astonishing. Ground level is often not identical in different places. Notebooks (their power supplies) can produce AC up to 90V above ground etc etc. And when plugging it all together it just gives a small spark or a more powerful current. The cause and source of all this clearly was not the DF, but the notebook (or computer) that it was connected to. And the fact that the operator got a shock should have made clear already that the notebook had been on a totally different voltage level without grounding or the same ground as the mixer.

I have seen units where this happened and the phones output stage got destroyed, together with the EMI parts around it. The unit usually still works (phones not, of course). Balanced connections never show this kind of problem, quite logical again.

It should be possible to service/repair it. Just contact your local distributor.