Standing and seating tickets?

Yes they will check and thats a good way to get yourself and your friend kicked out and possibly banned. Seat jumping and section moving creates a lot of problems for the venue. If your friend wants to go buy him a seating ticket. They are trained to spot people who do that and there are a lot of them. I work security at venues near where I live I have personally seen what happens when people try to do what you are talking about. Dont try it.

They'll check. I've been seated three times. Twice with set seats (to which we were directed) and once to general seats (sit anywhere). But you had security/ticket check/bag search upon entering the venue then another set to stop seated tickets going into the standing (and vice versa).

So they'll check and keep you in the section your brought tickets for. Unless ALL tickets were general admission and you could choose to sit or stand (or stand downstairs or stand upstairs etc but that depends on the venue)

Depends on the venue. Now a days it's almost impossible to get in a section without having a ticket.