Where do I draw the line..

It's not your baby. If they want to visit friends then that's up to them. Were you this involved in their life before the baby? I think you should take a big step back. Don't offer advice unless they ask for it. I'm sure it's coming from a good place, but this is their child, not yours,to bring up their way.

When my kids were little my MIL was horrified I dressed the children in black sock or dark trousers. Sometimes we would take the babies out later in the evening- that was wrong too. I didn’t listen.
They have to do what is best for them.

I just worry. They are new parents and haven't realised they need to think about priorities. They are literally visiting friends every day..

I think I'd ask them not to send me texts that might worry me, especially in the evening

Ohh that was a typo..
I was meant to say it is raining with wind warnings tonight.
They usually send me a photo of a night time when he is sleeping.

I think she means they send her a photo in the evening, rather than of the sky itself

What do you mean they ‘send a photo of a night’? A picture of the sunset? Leaving where? Going where? Are they driving, walking?

Assuming the baby is being kept safe, I’d leave them to it.