Hello, my name isXXXXX a Licensed Professional Counselor and would like to offer some insight into your concern today. Please remember, answers given on JustAnswer are information only and are not a substitution for professional counseling.
The jury on this topic is out for discussion and there are two hardcore groups of ideas, one stating that you should not introduce a child at all (with the most extreme stating that a single parent should not even date until the child is raised) and the other stating that for someone to be truly compatible with a single mother (or father), they have to be compatible with their child and familial obligations as well.
so the simple answer to your question is yes, there is considerable literature out there, though there is something to support both sides of the argument.
Like many situations in life, the answer to two extremes is sometimes best discovered somewhere in the middle, in this case, perhaps gradually talking to her son about the fact that she is dating someone new, before bringing him into his life after several months of steadily dating would be the best compromise.
while not all inclusive, "The Single Mother's Survival Guide" by Patrice Karst, and "Mom, there's a man in the kitchen and he's wearing your robe: The single mom's guide to dating well without parenting poorly" by Ellie Slott Fisher may offer her some insight into her situation
ultimately, while you seem to have your grandson's best interest at heart, pushing your daughter to not introduce her son to her new boyfriend is akin to me telling you to not think about pink elephants, once the idea is introduced it is almost impossible to resist.
I believe that you have done the right thing in explaining your concerns to her, and perhaps you can browse one of these two books and simply leave it out on the coffee table the next time that you go to visit.
Since you are offline I will exit this chat, but can return to further discuss this topic in Q&A style when you have replied. Take care! Anthony