A colleague scholar was an editor for my latest published article, in which I heavily used her work and cited it. She then sent me one of her papers, so I could review it informally and give her my comments on it.
At one point in the paper, one of my articles could fit perfectly as a citation. Is it ethical to suggest that she does cite my article? And how should I go about asking her? Directly in my comments to her paper?
If the reference is pertinent, it's perfectly fine to suggest it. "Suggest" is key, rather than "ask":
By the way, I've published a paper on the topic you're discussing in section three. Perhaps you'll find it useful. See attached.
If this were a "professional" review situation, that is, if you would review the article for a journal in a peer review process, then it would not be very ethical to suggest such a citation. Especially if the author of the paper might feel pressured by such a comment to add the reference to avoid the paper being rejected.
In your mentioned, rather informal, review setting is is perfectly fine to tell your colleague that you think your paper might add valuable insight to their work—she asked you for your feedback, this might include citation suggestions.
As Henning has already mentioned, be polite and not too pushy about it.
