I used a Neurotrac Pelvitone to help increase my pelvic floor strength, and my physiotherapist at the time was less than enthusiastic. However, the surgeon said that as I no longer had a prostate and my PSA was less than 0.1 consistently, I did not have cancer so the TENS machine use was OK. I have just re-read the instruction manual (or at least a version of it on the Internet) and there is nothing in the contraindications section about the use of the machine for people with cancer. In fact, one of the prime purposes (with surface electrodes) is to help with the relief of pain from cancers.
The only thing I can think of that might be an issue is that, for muscle stimulation, perhaps rather greater voltages are used and particularly with the (ugh) rectal probe, it is operating in an area very close to the prostate (or where it was if removed).
What is the actual wording on the instructions for your machine, and what model is it?