Dh and I have been married 15 years, two dc both primary age.
Due to work dh does all school runs apart from the odd occasion when I will go myself.
A few months back dh started mentioning one of the mums at the school was openly making it clear she fancies him. Never had a conversation but always looking out for him, trying to catch his eye etc. Our dc are not in the same years but their classrooms are nearby so plenty opportunities to end up having a conversation.
Dh said she has only ever spoke to him once and she was talking about the weather.
I didn't really think anything of it and it all got forgotten.
Fast forward a few weeks later dh mentions it's very odd how she acts like he doesn't exist when I'm with him on the school runs.
I thought maybe it's all in dh's head and she was just being friendly and he was thinking far too much into it.
A week or so after all that I notice dh now has her as a friend on Facebook as she commented on a wildlife photograph he had taken. I asked him why and he said she sent him a request and although he only likes family and close friends on his Facebook he accepted her so as not to seem rude.
Dh was mainly concerned with as to how she knew his full name, in the end we guessed she had seen him on the school Facebook page and just added him.
Anyway, I ignored all of this and again put it to the back of my mind.
After all was forgotten and it hadn't been mentioned again for a couple of weeks I decided to join dh on the school run.
I sat in the back of the car with youngest dc and our other dc in the front with dh.
We were parked up on the side of the road outside the school when the mum drives towards us. She didn't see me in the back as the windows are tinted, she only say oldest dc and dh in the front. She slows right down and pulls up alongside, rolls her window down and shouts "hello you" with a big smile. Car behind her started papping their horn as she's blocking all the traffic which made her move on. That is the first time I ever saw her acknowledge dh, she went out of her way when he wasn't even looking in her direction, yet when he's with me she refuses to look anywhere near him let alone say hello 
This made me realise it's not all in dh's head at all and call me paranoid/childish but the next time we went to the school I stayed in the car which you can see the playground clearly from.
I watched dh walk onto the playground, she immediately spots him and can't take her eyes off him, she kept looking over, smiling then talking to one of the other school mums who she's very close with and giggling like a teenager. She was very openly flirting with him.
All of this rather annoyed me but I kept my cool and ignored it ever happened. Dh deleted her off Facebook saying she probably wouldn't notice and he could tell I wasn't very happy about it all.
All went quiet again but it started playing on my mind after seeing what she was doing. I haven't been with him on the school run for a week or so now and she hasn't been mentioned until tonight when dh comes and shows me she's sent him another friend request on Facebook after she must have realised he deleted her. He declines it in front of me but told me she's continued giggling and openly flirting with him at the school.
I don't know how to handle this and it's making me feel like complete shit 
Do I do something about it? What would you do in my situation? Or would you just leave it alone?