Your friend is a big girl..............but won't be the biggest at the park. She has girth, but not a lot of height by the looks of it. I don't think it will be an issue here. I'll tell you why. I am 6'2" 300lbs. No kidding. I am built like a linebacker but with a bigger gut. I am big everywhere. Broad shoulders, thick legs, big arms, and am tall........etc. Of all the years I have gone to Disney I have never had a problem on a single ride. We just went to the Magic Kingdom last month. I had no issues with anything. I rode on Splash Mountain and no issues. Not even "just barely" getting in either. I was comfortable.
In the past at a size not much different than now I can only think of a couple rides that were snug but still not in danger of being unable to buckle in. Space Mountain is one of them, but I was fine. Actually my knees hitting the back of the seat were more the issue, so basically that came down to height. The other was the Matterhorn at Disneyland, same issue, knees. I was still fine on them.
Last month we went with our kids and I rode Dumbo with one of our kids. Plenty of space. Disney is not like other theme parks. Their rides are for everyone. I've been in that situation before where I have been kicked off a rollercoaster because I couldn't click in. Both times it was Cedar Point. I laughed at it. My friend told me jokingly I'd better go on a week long salad diet before I come here again. So I'll get the odd rollercoaster with my size, but never an issue at Disney or Universal. Tell her she'll be fine and won't have to endure the walk of shame.