I am sure she knows the place is a mess without being told so keep mum.
It has got like that gradually and my thoughts are that its beyond her now and she just cannot see where to start. The children keep her busy and she will be worn out at the end of the day.
Would it help if you offered to have the children for a weekend as a treat for them? I'm sure the children would love it and maybe it would give their parents a chance to make a start, working together with no distractions? I would not give that as a reason, just say it's a treat for the kids. If it goes well, it could be repeated.
It is so easy for a young mum to be overwhelmed with kids, housework, shopping, sleepless nights, cooking, never a chance to draw breath. Even if they don't tackle the house first time, maybe they will a second time. But she must know you are on her side so no criticism, just loving support. Good luck.