Hi, Jessica. Your friend is sweet like awesomesauce. My wife and I have taken our boys to Disney World when they were those ages, and we had the time of our lives!
There are loads of attractions that everyone can ride together, including everything in Fantasyland. Any attraction that doesn't have a height restriction is ridable for the whole family.
Check out this list to see the few attractions that the littlest biscuit likely wouldn't be tall enough to ride. I think your friend will be surprised with just how many attractions are fair game. And, in the event that the height restriction keeps the 18 month old kid out of contention, you can take advantage of the rider switch pass: one parent rides with the 4 year old while the other hangs elsewhere with the 18 month old. When the parent with the 4 year old is finished on the attraction, the 4 year old gets to ride it again with the parent who sat out the first time around.
Midday breaks are key for both parents and kids alike.
In addition to a sit and stand stroller, one of the best things we brought with us was a toddler backpack carrier (REI Piggyback) for our 18 month old. He was an active guy and had a 5 minute expiration period when stationed in the stroller. He was happy and content in the backpack carrier. Napping in a stroller was unheard of for him, but he would check out in the backpack carrier without a problem.
Please let me know if I can help her with any other tips and tricks!