She's in denial. It's hard to hear your child may have issues.
I wouldn't compare his speech to that of another child, though. Literally no two children are the same.
I had a 4yo who was like that- I could understand 20% of what he said, if it was quiet and he repeated it. Mom swore he was fine. I ended up terming over it, as it was causing HUGE behavioral issues. No peers could understand him, either. He was ending up very socially isolated.
He went to K and I heard he was in spec ed.
That being said, I would do an ages and stages form and send home a blank one. For ALL kids so she doesn't feel singled out.
I find MOST parents don't see faults/delays in their own children easily.
I miss when pediatrician's used to spend time at physicals with patients, now it's so hurried they take the parent's word for it.