Hey,
While I'll admit that I'm just looking at pictures and it may seem different being there, and by no means do I mean any offence... (good to see anyone willing to put in some effort for others as well as themselves), but I would say that some of those jumps/landing ramps look to be made of "minimal" material...
Personally - if they were on my property - I'd be bulking them up considerably... Take it or leave it, but despite your mates being your mates - if one of those jumps was to break on them, you could well have a legal case on your hands...