Yoel Romero hit 185lbs on his weight day (well, most of the time) he's the same height as me 5'10.
Sean O'Malley is 6' and he hits 135lbs on weight day.
Keep in mind they actually weight 10-15lbs more on fight day. These guys are top athletes in the world,they play with their weight alltthe time and compete on the greatest stage in the world.
I know a guy who weights 215lbs, no apparent muscles, no apparent fat, he's 5'11. You'd see him on the street you'd think he's 180lbs.
Just to say, weight is really differant from an individual to another one. It's not a set thing and there's metabolism/health reasons for that; during a human lifetime his body weight and muscle mass will constantly change for many many reasons. 5'11 145lbs is very lean I still doubt it's dangerous, especially if he has not much muscle mass.