from the beginning she has tried to portray my son in a bad light (he's far from perfect but bearing in mind we were in lockdown, he'd just lost his father and hadn't seen the baby for almost a week after she was born) is it surprising he was wrecked?
It is very understandable that your son was in a mess in those circumstances. I think it would be wise to talk to him about any possible sources of the "dangerous" comment from DIL ... when upset what did he do/say etc.? What happened when he saw the baby etc. ? Not accusatory atall, just working together to try to understand where she is coming from and helping him to work out a way forward for him with getting access to his child. I also do think that if he tells you anything about what he did when upset it might give you insight into the problem from your DILs point of view.
She is always coming out with strange things- on my visit to see the baby she made a thing of saying if they split up she wouldn't stop me seeing the baby. I just looked at her and thought what an odd thing to say when you've just had a baby!
That does seem strange but does suggest that she may have had the split in mind at the time. Has your son told you why they split up?
You mentioned a SW (social worker, I assume) which suggests that there have been problems in the family for a while ...did Social services become involved after the baby was born? Why? What happened?
You really don't have to answer those questions on here if you don't want to, they are things that you might like to consider for yourself though.
If you do decide to contact DIL then I would suggest just a friendly "Hi just saying touching base, and wondering how you are. Hope that you and * are ok. It would be lovely to see you both, fancy a coffee?"
So you wouldn't be engaging in talking about your son and their relationship but getting a feel of how the land lies. If she agreed to meet for coffee, play it as she wants to maybe? Ask about your granddaughter, how DiL is getting on etc. If she starts talking about your son, you can listen and (if appropriate depending on what she says) just say "I am sorry that this has happened. I know he misses (their child)." and go from there
All of the above though depends on what sort of relationship you have had with her prior to their split.