I'll ask for clarification.
If all altimeters fail, the crew is unable to reliably detemine their level. How should i separate other traffic? Assume, that a/c with malfunction can be at any level between gnd-460 (top of our FIR), or maybe ask the pilot to guess their altitude if in vmc? He/she wont be able to tell FL360 from FL380, but FL380 from "somewhere around FL150" should be doable (with huge error, but still it would allow me to treat them as being under any other traffic")?