Could you suggest that when she is not well you all go to see her? If you made it just a day trip that would cut down on the length of time she was contributing her cold germs. Get DS a "special spoon" of his own to use, so that she is not feeding him tasty morsels with the cutlery that has just been in her own mouth.
She must be very determined to see him if she is willing to travel two hours fortnightly to see him, especially when she is ill. Perhaps you could suggest that she is not doing her health any good to travel such a long way when she is low anyway.
Have you explained your concerns to her in a very clear but kind way? If you can get her to see the risk to DGS' health she is creating, she would surely understand? Also, if you are catching her illnesses that easily, are you sure your immune systems are up to scratch?
Grandmother of our toddler lives alone, 2hrs from us. Our DS is entirely central to her life. She comes to stay for 4 nights, twice a month to see him. But she keeps coming to stay when she's ill, with colds, nasty chest infections, stomach bugs. She is ill very often (immune problems). And cannot understand hygiene i.e. keeps feeding DS bits from her plate, however many times we explain or even get angry. It's just happened again. She came, very unwell. DS and I are now ill too. We have suggested only coming when well; she says she'll hardly ever see her main joy. I don't want to damage their relationship but I'm fed up. Could you give advice from the grandparent side of things?