Mrskeats may I suggest you think in advance about what you can say and plan it out so that you do not go far and say too much all in one go.
And of course read her reactions as you speak because blurting it all out could be very heartless if she is very offended.
If she secretly feels quite desperate about it she may be more willing to discuss it. The fact she lets you in to her house makes me think she doesn't feel it is so bad... from my watching of hoarding and cleaning programme (I love them) people can sometimes become very unwilling to let people into their homes if they are very messy and cluttered.
I do find watching decluttering and cleaning programmes make me want to clean more!
There are loads of decluttering thread on Mumsnet. take a look to get some ideas. There are some of Marie Kondo, the Japanese expert in tidying and decluttering, threads here.
If I were desperate I think a friend offering to help would be brilliant.
So please do think about positive solutions to 'offer' so you are not just dropping info on her and then leaving her to it (e.g. I could help you with XYZ, we could get the kids involved too in some simple tidying up etc).
Start small, one job at a time, or one small room or one small area of a room.
Offering support and encouragement could be just what she needs.
Be careful you do not get sucked into doing everything for her.
Good luck.