Trust me, it's not. Whether he can use it or not depends on the FA's. Especially in the equipment with larger FC cabins, the FA's will often ask non-FC pax to leave the FC area and use the lav in Coach.
One of the perks of flying FC is not having to wait for the use of the lav. Or at least not in a line.
On my last FC flight, the FA's were allowing non-FC pax to use the FC lav, but not to line up for it. And if a FC pax got up to use it, they would ask the coach pax to return to coach. In other words, if it wasn't being used, it was available to anyone, but if a FC pax needed it, they got first crack at it. And they didn't allow more than one coach pax in FC at a time for it.
I've been on flights in FC where the FA's let anyone up front, there was as long a line for the FC lav as there was for the coach lavs. I've also been on flights where the FA's didn't allow anyone not in FC to use the lav, even though coach was full and FC only had three pax.
When I'm flying coach, I do not use the FC lav. I'm sure others will disagree.
Anne