To be helpful, I offered the following advice…
From a coaching perspective, the first question I encourage you to entertain is, "What internal dialogue is leading you to conclude that this is a delicate issue?" It is an important question to consider because how you FRAME IT IN YOUR MIND will DIRECTLY IMPACT HOW YOU FRAME IT WHEN DISCUSSING IT WITH HER.
In YOUR mind, what is delicate about this situation, and what are you most concerned about when having to address it with her? If I took an educated guess based on how you posed the question, my guess would be that you don't want her to perceive this as an evaluation or judgment of her as a person. If that is correct, my next question is, "Is that what this really is, or is it simply a skills gap that's been identified?"
Part of what HR is responsible for is IDENTIFYING SKILLS GAPS and PROVIDING THE RESOURCES NEEDED TO CLOSE THOSE GAPS so that team members can reach their fullest potential and achieve their professional goals. Since speaking and writing effectively are important components of her communication skills, this is an opportunity for her to enhance her communication skills so that they are in alignment with and supportive of the value you know she has to offer. When sharing your observations and feedback with her, I encourage you to stay focused solely on that.
In addition to that...Be less concerned about the judgments made by others and how it might hold her back and, instead, shift your focus to the growth opportunity you believe she is deserving of and then align it with what matters most to her. What are her priorities? What does she care most about? What does she want for herself and how will taking advantage of this growth opportunity help her achieve that?
Also, you might consider going into the conversation having already identified potential resources for helping her close this gap. In doing so, the subtle message you send is that you are less concerned about spending time critiquing her communication skills but rather, more focused on helping her acquire the resource she needs to help her elevate them.
Lastly, after sharing these potential resources with her, engage her in the process of identifying and selecting the resource she feels would be most effective. Ask questions to better understand how she likes to learn and what has been most effective for her in the past.
I commend you for caring and your willingness to address this with her.