There are no weight restrictions, or we have never heard of any except that you have to be at least 90lbs to hit the class 4 and 5 rapids, and it doesn’t sound like that is a problem. As far as physical strain is concerned there are usually 8 to 12 people in a raft her input will not be missed. Just have her in between two stronger paddlers. I have been on several trips and there are usually at least 3 or 4 people in the raft that do very little paddling. Mention it to your guide and he will make sure to put her in a position that is insignificant. Mom if you’re reading this “Go for it!” you won’t regret it.