If he is senior and friends with your boss in such a way that you can't talk to them (your boss), then he is effectively your boss since he has power over you.

If he is senior and friends with your boss in such a way that you can't talk to them (your boss), then he is effectively your boss since he has power over you. You may believe you are smarter than him and a better developer and even a better human being... it doesn't matter because you work somewhere that allows some nepotism and are willing to continue to pay "Senior" developers who aren't that good. If you like the job/company, suck it up and just ignore this person whenever you can (kill them with kindness and patience when you can't) while improving your position through adding value and doing your job better than can be ignored. If you don't like it, then keep that resume shiny and make sure you do a little more vetting of your employers in the future.