Rides aside, your friend will have to consider the incredible amount of walking he will be doing in the parks. I'm about 5'7" and weighed about 180 on our last visit (lost 20 lbs since then). My desk job doesn't give me a lot of time on my feet. After 2 days in the parks, my feet looked and felt like two pounds of ground beef, with broken blisters, hot spots, etc. And the heat can take a tremendous toll on you if you're not used to it.
He should get to a doctor before the trip to make sure he can handle this kind of activity. It will be much more strenuous for him because of his size, and he will be prone to bone spurs, knee problems etc., just from the walking. Make sure he has comfortable, well fitting shoes, even if they're expensive. Make sure the Doctor checks his blood glucose levels. If he's developing diabetes, the strenuous activity could just push him over the edge.
Sorry to sound like a party pooper, but my clinical training leaves me worried for him.
If the doctor gives approval, he should go and enjoy himself, and to heck with what other people think.
As for me, FIVE WEEKS AND COUNTING!!!