The way you approach it can make a difference. 2 of my 6 kids were "on the autism spectrum," (Asperger's) but now you'd never know it (well, most people wouldn't). I'm so grateful for a friend who told me that she saw some signs in my toddler that she'd recommend getting him checked out by a local early access program (that specializes in early intervention for all sorts of childhood issues.) My pediatrician didn't see anything out of the ordinary, but my gut told me to follow through on her advise.
It helped that she told me, not out of fear, but out of a sincere knowledge that early intervention can make such a difference for so many children, and IF he needed it, it'd be good to know. That's all. It was a "maybe it's nothing, but I'd recommend you look into it, just in case it could be helpful" type of thing. And boy, was it helpful!
When he was younger, he would often seem to be looking right through me (but not always, just a lot of the time), and he was SO particular about things, had some sensory issues (couldn't stand to walk on grass bare foot, or be outside if it was windy, be in loud places, swing, or ride in a shopping cart), was obsessed with cars and mechanical things and would shake his head and shoulders back and forth (as if simulating rocking) to fall asleep. The terror in his eyes when he was in a shopping cart or in the wind made me know his tantruming was because he sincerely couldn't handle it. There were just a lot of little things.
Now, he'll be entering a regular kindergarten class next month and he's doing great. Would he be doing as well without the little changes and therapies we provided for him? Maybe. But I SAW how deep pressure would immediately calm him down in the shopping cart and at times he was overloaded. I SAW how working with him on the trampoline helped him develop a better sense of being grounded. I SAW him overcome him obstacles with guidance and I'm so glad we were able to provide him with that while he was so maleable.
Talk to her. Just be optimistic. It may be autism, it may be less severe, it may be nothing. No need for fear. Just get checked out so you have options, IF needed.
I think what upsets people is when parents "freak out," insisting that something is wrong with their child when they think it's just normal growing up. I'd say, there is no need to be fearful and overly anxious. Knowledge is power, though. It is GOOD to get honest evaluations, listen to your gut and be proactive. It is not good to suppress feelings or reason them away out of fear. If it's nothing, great. It's nothing. Although, I'd also say to encourage the mother to consider getting him checked out, but then to listen to her gut no matter what. (Not all doctors are as convinced of the prevelance of autism-related disorders or the impact of early intervention.)
As mothers, we are blessed to have other mothers who help support us with our children. Kudos to you for caring for them.