It is an ME system, and should be evaluated under IEC 60601-1-1 (2nd ed) or IEC 60601-1:2005 Clause 16.
There are two basic principles:
(1) electrical devices should comply with their appropriate standard, e.g. 601-1 for medical device, 950 for the external PC
(2) the set up should not cause leakage over the limits in 601-1 inside the patient area in normal condition and with any single earth wire broken
The second part is a bit of a problem as it is really a case by case situation.
For example, if the external PC is Class II with low leakage, maybe no action is required. If the medical device and external PC both are Class I (earthed) and plug into the wall separately, again maybe no action is required since there are effectively two separate earths, breaking one is not a problem.
But if the medical device is Class II (double insulated, no earth) and the PC is earthed, then a broken earth in the PC could result in high leakage going to the medical device and hence the patient.
A separation device simplifies things. Other possibilities are using an isolation transformer for the PC, or adding another earth to the wall.
In any case, most manufacturers simply provide a warning that the person who sets up the system should follow IEC 60601-1-1 (or IEC 60601-1:2005 Clause 16). It's a bit of a cop out, as most users wouldn't know what that means.
In truth it's not that critical for most medical applications. There are only special cases where it would really be important. Leakage from a PC can be high enough to hurt, but not stop your heart if applied external to the body. Important cases are for example if a surgeon is performing a delicate operations, where shock could cause them to move quickly (from pain) and then injure the patient (unintended action); catheters which might direct the current straight to the heart, or open heart surgery.