I'd write an anonymous letter but finish it as if it's possibly from a few of the neighbours e.g. kindest regards, your concerned neighbours.
Re the content of the letter I would suggest that she looks into getting a relative/friend to walk them once a day REGULARLY so that it breaks up the day for them, (personally I have a dog walker for the days we're not at home and he wears out my dog!)
Then, suggest that if that if that is not an option she should employ a dog walker, (I know she's on social but it needs to be suggested)
Then hint that if the incessant barking does not stop then one of the residents will have to start taking detailed logs of when the noise is happening, and for how long etc etc, then post that to her and then if she still doesn't do anything (i.e. have them rehomed) then complain through the proper channels to the council. As far as I'm aware I think here in the UK they have to take them to a dog pound, so it shouldn't be so glum for them.
Regarding the RSPCA you could ring them anonymously and ask their advice on the dogs needs, eg stimulation/ not getting bored, then see what they suggest. They might think they have a case anyway as the dogs are clearly unhappy. Let's face it, they would not just show up and take the dogs away from her, they would give her advice and opportunities to improve the situation.
Good luck keep us posted.
Cmitchell