Bathroom Sink Faucet Repair

[Josh Cook]

My renter/friend put a new aerator on the faucet and then the bathroom sink started leaking. I ended up having to call a plumber who fixed it. The tenant/friend didn't put the washer on the aerator which is why it was leaking. The sink was working fine with the aerator before and now works fine without leaking since the aerator is properly installed with the washer. 

The plumber cost me $75. Do I pay or should I make him pay?

Thanks,

Josh