You are right about the time; April 4 is Palm Sunday, so she is planning on going the week before Easter. She may not be aware of this, but the week before Easter is one of the busiest weeks at WDW. We have been at WDW over Easter before and the parks will be packed (in fact, for Easter Day, MK usually closes to guests who are not staying at one of the WDW resorts early in the day). Because it is so busy, we have found the CMs to be a bit less accomidating to people with special needs - it's not that don't want to be helpful, but with so many guests in the parks, they are just concentrating on keeping things going.
If they can get to the parks early (like right after opening), they will usuallly find smaller crowds and shorter waits. Because the parks are open later, some people stay out late and don't get to the parks right at opening. I can't get to the parks that early (I am not a morning person and my yougest DD, who is disabled, requires too much getting ready in the morning), but my DH, older DD and other members of our party have gone at opening and found they were actually able to get on most of the Fantasyland attractions with short waits early in the morning (like some they could walk right on and went on twice without waiting more than 5 minutes each time).
Because it's busy, WDW doesn't typically offer a lot of special prices for rooms during Easter/Spring break (they can fill the rooms without doing any specials). In fact, she is very likely to find that all the rooms at all the AS resorts are full, so even getting a room at all may be a big problem and getting a room at the Toy Story area at ASMovies may be totally impossible (that is a highly requested area in any case since many small children are wild for Toy Story).
I know she wants to go NOW, but going a few weeks after Easter is likely to make their trip a lot more pleasant. There is a noticeable difference in the crowds the farther away you get from Easter.
If you haven't already, look at the disABILITIES FAQs thread. There are 2 parts that will be helpful.
The first is the links to the official Disney website's disability pages, which includes the text portion of the Guidebooks for Guests with Disabilities. Those have (kind of general) descriptions of the attraction, which include warnings for those attractions that someone with heart problems should be cautious of. It also includes information about restrictions like height requirements. You can get a printed copy of these Guidebooks at the WDW hotels and at Guest Services in any of the parks. The park maps are also coded to show which attractions have restrictions/warnings (they have a red triangle icon for precautions).
The other part on the disABILITIES FAQs thread that will be helpful is the information about Guest Assistance Cards (to avoid typing so much, we call them GACs).
GACs are not a "benefit" for disabilities; they area tool to help the CMs know what sorts of accomidations the person needs. Although some people feel more comfortable having a doctor's note when they ask for a GAC, you actually don't need one and the CMs at Guest Services may not even want to look at it. If she does get a letter from the doctor, it should talk about things the child needs; not the child's diagnosis or problems. For example, saying a child has heart problems doesn't really tell anything about their needs. Some children with heart problems have no restrictions and don't need any accomidations; others may have a lot of restrictions and have a lot more needs.
What you do need is to think about the child's needs for accomidation and ask for assistance that will meet those needs. That might be things like using a stroller in lines, as Sue & Co. mentioned.
