Melinda,
First of all, what a good friend you are to be so concerned. Oddly enough, about 18 months ago I was in a very similar situation. The child in question was 3 1/2. I often had him in my care and thought "something is just off". He wouldn't interact with my son, he rarely spoke, when he did it came out like silly baby talk and often times his fingers were in his mouth. He was and still is, very clingy, had very irrational fears (like so terrified of my dog that he'd almost vomit - which I know kids can be scared of dogs, but this was such an over the top reaction) and the list goes on and on. Now nothing is really different with this child. He's now started spelling large words backwords - it just makes you wonder. How does that happen?
At any rate, I tried to very gently bring up my observations with the mom. She wasn't completely unreceptive and often times, I'd just say "hey, may be bring that up with the pediatrician". They did go on to have him tested at which time the child was seen by a psychologist who said he was unable to determine at that point. They were referred to a speech pathologist and were supposed to take the child to occupational therapy. After 5 sessions, the parents decided it wasn't working.
My thought was that this wasn't going to change overnight and didn't think they were honestly ready to deal. I know it's got to be so hard for a parent to sit and wonder if my child could have something like this going on. I know it's gotta be a normal reaction to deny any difference in their child. I know my friend went thru the same denial process.
I sort of let it go and thought to myself that there will come a time (like when he went to school) that this would definitely re-surface. Well, yes, he's having difficulty in school, socializing, speaking, and now the mom is starting to actually accept that this little boy could be mildly autistic. I don't know and haven't read a ton on the subject, but when our Mom radar goes off we can't help but wonder.
My advice to you is to sit it out and hope that this little boy is a late bloomer. Time will tell and the parents will deal when they're ready if it's something more.
Hope that offers you something.