Just for once, I rather like Tetjam's idea.
I'm not sure what 'compromising' Alponce thinks has occurred. It is unlikely he has compromised the buildings structurally (since he has erected an RSJ to support his walls above). He has not compromised fire spread integrity because a wall is still in place (yours). There are now rules about noise transmission in Building Regs (mainly brought about because folks in flats were fitting wood block floors and the noise transmits downstairs). But these rules apply to a new build - and I'm not sure how it works if one makes a Building Control application for an extension.
He will have had a make a Building Control application if he has put an RSJ in place - no ifs or buts - it is a must. Some people forget this when doing extensions - to demonstrate the structural integrity.
You could play this one of two ways: -
1) Ask him, or find out another way, if he has made a BC application. If he has not casually drop into the conversation that you intend to talk to BC about it unless he bricks it back again. Making a retrospective application now (after the event) would be costly for him - but then there are costs in refinishing his new bathroom wall afain (tiling etc.?)
2) Go to BC anyway yourself and try forcing it on the noise transmission issue. They will advise you whether this is admissible. As I indicate above, I am 100% sure on the rules - it's called Part E of Building Control Regulations.