The claim "Dark Chocolate can be beneficial for people with diabetes" is true, and there is a lot of evidence and findings to back it up.
According to an Abbott news article, "Dark Chocolate contains polyphenols..." which "...may improve insulin sensitivity, or how well insulin works in the body". This is because polyphenols contain antioxidants in them that are protectors of the body when it comes into contact with dangerous molecules. Instead of making a diabetic's blood sugar heighten significantly, the dark chocolate is actually helpful.
In another piece by PubMed Central, it explains how "the vasodilatory effects of cocoa can also improve insulin sensitivity mediated by endothelial function". As cocoa is the main ingredient in dark chocolate, this means that eating this chocolate could actually help deal with diabetes as a whole condition. It won't just make someone feel better, but it may actually lessen the severity of their condition over time: "sustained consumption of cocoa over long periods may affect insulin resistance to a greater degree than single doses of cocoa".
Lastly, the Harvard School of Health compared dark and milk chocolate's effects on diabetic patients. They found that even though the chocolate's have "similar levels of calories and saturated fat, it appears that the rich polyphenols in dark chocolate might offset the effects of saturated fat and sugar on weight gain and diabetes". This means that consuming the chocolate may actually reverse some of the symptoms of diabetes, and maybe improve the quality of someone's life.