I wouldn't call it "irrelevant". If he did, then he got value out of the session, even if not the full value seeing as a period can put a damper on what kinds of activities you can engage in. But still, value. Even if he didn't get off, he got some value. Though, considering the end of a session is far more valuable than the beginning, he got less than half the value. How much value exactly would depend on the person doing the valuation. Chances are the SP thought it was full value, and the OP thought it was no value, both of which I find unrealistic. Good on the agency for whatever compensation they offered.
This is just one of those unfortunate parts of the hobby. No sense making a big fuss over it.