I think we can safely say that the differential is rather long and that almost all possibilities are plausible.
Was it an arrhythmia due to ischemia that triggered the syncope? Possible.
Could it be changes associated with a head injury? Its possible - need a CT to be mostly sure (subarachnoid could possibly not be seen).
Could these findings be associated with hyperK in a dialysis pt.? Again, it's possible, though I'd place this a little lower on the list considering that she hasn't missed a session. (Assuming she wasn't down for a prolonged period of time.)
Could this be completely incidental; a normal for her? Absolutely (need an old ECG to know).
I recommend follow-up, if possible. but if they tell you her K is 6 (or 7 or 8 - considering that ECG changes are variable), I'd still be hesitant to call the changes as being secondary to hyperK, unless it reverted after the K was returned to normal.