If her DH spoke to you then I would raise it with him on his own. It is difficult to know whether to do something or not do something. What I would say is when I realised my DS had autism it was scary and devastating. its like a bereavement (my child actually went backwards and lost lots of skills and it was like my baby died and I got another much more shut off child who stopped speaking / looked through me / stopped calling me mum).
There is a grief reaction - with perhaps the same stages - shock, anger, why me, denial. Can take a long time to get to acceptance and then it comes and goes - days when its still hard to see other children doing things mine can't.
When I started to raise concerns I found it really unhelpful that people brushed them aside - the 'we all have days like that' - 'or my child does that too'. No you don't you don't have a day when your child isn't learning or developing properly. People often feel they should minimise the problem (because they feel uncomfortable and don't know what to say) but that can leave the parent feeling really isolated and stop you talking to that person again.
I wasn't depressed - although counselling might be helpful I cannot imagine how I could have fitted it in. DS needs were relentless. He had no awareness of danger and I couldn't leave him. His sleep was awful. I was worried sick, isolated, grieving and sleep deprived. Those probably all made me seem like a less effective colleague / less happy person / moody cow. I didn't take it out on DS but I'm sure DH and I had days when we supported each other and days when we snapped at each other because we had no-one else to turn to who 'got' it & we were totally knackered.
Chances are she cannot leave her DD because her needs are so much higher / different. I rarely leave DS with a sitter - he hates it - those we do have are specially trained. Dh and I go out maybe once or twice a year. I never leave him overnight.
I know you mean well but it must feel like a chasm is opening up between her life and yours (the life she expected)
The best thing a friend did for me was listen. Not minimise, not try and get me to do normal things or go out, not give advice, not judge or analyse whether I was depressed or negative. Just when I said I am worried about DS she simply said why are you worried - and let me talk while she listened. (she's now a trained counsellor) 
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The DH could see the GP on his own and would get a referral to developmental paed. Or can self refer to portage (play therapy), speech therapy, hearing test or ask HV to refer. Poss to physio too. The child will be under Child Developmental Centre at local hospital not under GP. I haven't seen DS GP for years. I would speak to the DH again. Play therapy or physio doesn't sound as scary as medical stuff. Maybe just say you are worried too and wouldn't be better to see if could benefit from play or speech therapy / physio / get hearing checked etc. Many areas have drop ins for speech therapy or portage and the DH could just turn up. He might not want to upset his wife but he also is clearly wanting do best thing for his DD.
I just about held it together at work but would have fallen apart talking about my DS to someone at work. I used to hold it together in front of my other kids, drop them off at the school gate and literally sob all the way home. When my FIL was dying of cancer my MIL would sit in the car and scream and then go back in the house and look after him as normal.
Everyone is different and you might just have to put up with the fact she is struggling / grieving at moment (and defend her when colleagues bitch about her). She needs a sympathetic ear and to find her own path / solution and feel supported. I dealt with it by getting stuck into finding the right help and found once I had learnt some strategies and could see progress again things got better. i got hope back. Not that things will ever be easy - but that DS would improve and we would manage.
A few years later my DS2 was diagnosed with Type 1 diabetes and has to take insulin for the rest of his life. i didn't even cry. It was a quick diagnosis and he got the right help immediately. Totally different than autism where it took 5 years and 2 appeals to get the right help. Yes it sucks but there is a plan and its manageable, he didn't die. He can have a good future. Other families would have struggled with news like that but to DH and me anything not cataclysmic doesn't even touch the sides anymore.
Try not to think of her as mean / negative / depressed. Think about how you would feel if you lost someone close to you and were grieving. Would you want people gossiping about how you'd had a personality change?
I'm not trying to give you a hard time. If it helps I look back at times before I became a parent of a disabled child and cringe at some of the useless / crass things I have said to people because the truth is you just can't imagine what she feels or what she is going through until it happens to you