It doesn't sound like they are abusing her, just neglecting her some. If the dog has food, shade, water and shelter from the rain, they aren't legally abusing. I'm sure they would like to spend more time with her since they let her in when they come home at night. Maybe volunteering to walk her while they are at work would ease your mind and give her some exercise. You don't say if their yard is fenced or if she is tied up.

An insulated dog house would help for both heat and rain.

It sounds as if they are doing the best they can if they both work. Hopefully she has enough room to potty and get away from it to lay down. A lot of dogs do fine in these circumstances.

I don't think another dog is the answer. They probably think a smaller dog will need less time and can be kept inside. The size of the dog is less important than the training it gets.