Okay serious answer, start a dialogue about it. Ask him why he wants to be a cop, genuinely try and understand what his reasoning is. From there, you have three options
2) poke holes in his own reasoning. Use the tin man argument, state his position on it better than he can and verify that’s where he’s coming from. Then you can poke holes in his reasoning all you want
2a) he’s persuaded(might happen might not)
2b) he still becomes a cop and you stop being his friend(harsh but sometimes needed)
2c) this is just option 1 again, you don’t have to agree with or support it
Simply, try talking it out. Some people approach going into the police as protecting and serving their community. Those people are naive and the police actually don’t have to protect someone unless they’re in their custody and even then they don’t. Never mind the good cops are the ones that quit early because of how corrupt the internal culture is what with the fact cops very much fail upwards, but I digress
Figure out what the hell he’s on about and go from there