If your DP was raised by these GPs, perhaps there may be something that HE can say and they will listen to, rather than hearing it from someone else.
BUT, OTOH, he is hours away compared to minutes for everyone else.
So he needs to tell everyone else to stop providing the treats as that is part of his DGM's mismanagement of her diabetes, and help his DGF get things back on an even keel.
I am torn in this instance about going today if he will be going over the weekend and others are on hand. Can he ring DGF himself and see what story that brings? Would the DGF downplay it because your DP is far away?
I think you may have to suck up this one, as DGM is actually ill (chest infection) and that may bring complications. But, once this crisis is over, your DP needs to make sure that the wider family know that, between the distance and the imminent arrival of his DC2, he will not be as available as he has been to deal with such crises. And that they will need to get better control over the issues themselves - including finding other "treats" that do not involve things that are actually dangerous (he could even say "poisonous") to his DGM because of her diabetes.
I agree though, it is DGM's responsibility for what goes into her mouth, not your DPs. And he needs to make sure that his own DPs are aware of that and to stop laying the guilt on him.
(In my case, I live a similar distance away from DPs and DMIL, and DH is "on call" a lot over the past year while DFIL was ill and passed away and as DMIL has found it very difficult to cope, my own DM has got her diabetes under control and has 2 of the 6 DCs living - not walking distance but about 15 mins away by car - who are able to assist when needed, so I understand some of the worries and problems involved).