You didn't provide that much information to work with. Local LANs are very often set up as a "normal" servers, this means that the owners of the LANs have their own private dedicated machines on which they setup normal servers which are very similar to these which you see in the community servers tab ; surf servers, 1v1 servers, deathmatch servers. This means that they usually won't have any extra protection apart from the build in VAC protection on the servers which is safe as LeagueCheats fully supports VAC.
Owners of the LANs can also set up offline servers where the VAC would be usually disabled, but I don't think they would do that as they would not have access to different plugins for servers and won't be able to control the game fully. This would also be very stupid of them as they would have close to non security on their servers and I guess a lot of people would be injecting.
There's also a chance that LAN like this, as you stated 5k prize pool is pretty big might have some extra security and no one can really help you then as we would have to know what type of security they would be running.