A raised bed staying too wet is a little unusual, it should be draining faster. What did you use to amend it, did you line with plastic underneath?

It's normal for oldest rhododendron leaves to turn color and drop - leaves three years or older so lowest. How does the rest of the foliage look colorwise, are you seeing any limpness/yellowing to the newest leaves?

The longer it is moist and hot the sooner it will die. There is no guarantee that you can save it. Most root rot diseases are fatal. If the plant is just reacting to the moist conditions, then moving it soon could save it.

The soil mixture for planting rhododendrons must be one that can be well drained. In nurseries, they use a mix that is rich in bark dust, but they water often. In my garden I use a mix of 50% native soil and 50% peat humus which I buy in bags at my garden center. A lot of people like to use peat moss in the soil mix, but it tends to hold water a lot and gives some people problems. Nurserymen don't like it. Homeowners still use it.

Here is a link that might be useful: How to grow rhododendrons and azaleas.