Care to explain how that actually sorts the problem? 2 months time, a kid jumps up and down in excitement on that spot and the pipe underneath gives out ? Not sure how a cut out floorboard will help the situation. If I had been called to sort the floorboard, cutting it out to accomodate a pipe wouldn't even be an option I contemplated. Always cure the problem first, not the issues it causes.
The problem is with both the pipe and the floorboard. If the pipe was lower than the joist I still wouldn't be able to screw down the existing floorboard due to it not wide enough.
That I understand. But it is no use securing the floorboard if the pipe is still too proud. Unless you can cut just enough of the floorboard away to give the pipe clearance and leave strength in the board. If you can't achieve both, then the option of thinning the floorboard is of no use, just a bodge that will resurface sooner or later, possibly with worse consequences. Once the pipe problem is resolved you can then work out how to support and fix the board. Probably by rerouting the pipe you can have full access to proper support for the board.
Assuming you can fit a routed board over the pipe. Then lift the boards either side and fit the routed board over the pipe and make up the difference either side with narrower boards.
If it's just one floorboard that won't go down, then logically, the pipe is low enough either side of this board. Just notch out the Problem joist to allow pipe to go down, wrap a bit of hair felt around them, then put the board back down. They don't have to be fixed right at the edge.