A friend of mine was in a club with his car parked outside. Police used flashlights to look inside vehicle (I am uncertain why) and claimed they saw a firearm sticking out from under the seat. They waited for my friend to come out of the club and walk toward the vehicle and arrested him. He never produced keys or registration which would indicate ownership of the vehicle nor did he indicate that the vehicle was his. I actually belonged to his mother. They towed and searched the vehicle and charged him with constructive possession as well as the possession of a firearm after they found the gun and also some marijuana.

Was this legal?

Police can conduct a plain view search of a vehicle. The issue here is why they were doing so in the parking lot of the club since it is private property, unless he was parked on the street or public lot. There is also an argument to be made as to whether it is "plain view" if they are walking around with a flashlight at night and there is no sign saying vehicles subject to search. Go get a good criminal defense lawyer asap.