I would be very honest but kind and let them know these behavioral issues have become a problem and tel her exactly what you say here telling her you fear for the safety of the dog. It is not only perfectly fine to commend training for a client's dog, my clients actually appreciate and expect my opinion on the matter.
I am lucky to be a certified and experienced animal trainer and behavior modification therapist so I can do the training myself and charge for it but if you do not feel comfortable being the trainer, you can always find a trainer you like in your area and recommend them to your client. Make sure they are using mostly positive reinforcement methods, see them actually working with a dog and if you have not used them yourself, pick one who has been referred to you by people you trust who own well trained dogs.
You can also find good websites like https://positively.com/ full of super good info on all sorts of training and behavioral stuff and actually send links to articles regarding crate training or any other thing that needs work. The more info you are able to give your clients about their dog, the more the trust you and feel how much you care for their dog.
But if you are no longer comfortable with having this dog in your home, it is perfectly fine to tell your client, it is important that you have a good rapport with the dog or it wont be pleasant for anyone. Simply tell the owner that you do not think you are the right sitter for their dog and that you feel someone better suited for his particular needs would be a better fit. You can find a few other sitters who would be qualified and willing to take your client's dog on before telling your client so you can give them options when you stop taking in their dog.