Del can't kiss her, she might die

I think he can...

I think he can, my guess is that the dude who gave him the medication said that he can’t kiss her not because it’ll kill her, but instead because he likes her

No, remember his medication? It's lethal to a normal being, it's labeled as poison and he's been consuming it. Which means, every bodily liquid from his body is almost like poison and saliva, is a bodily liquid.

Yup he is jelly

Yup but she is his antidote.