In my experience, a new user that is bursting with participation soon after they join is very receptive to feedback, with the cavaet that the feedback must be plain, visible, and friendly to them.
If you take the initiative to open a line of communication with the user, you will be better served by doing so in a way that they will be notified that they have been communicated with. If your platform supports private messages or something comparable that will notify the user, these are usually the most direct means of communication.
While attentive, these users can also be fragile and unwelcoming to feedback that isn't construed as helpful or friendly. Make sure to have a friendly demeanor when constructing your message -- otherwise, your help may be ignored or resented.
It sounds like the user in question isn't a totally lost cause, which is good. When constructing a message to address your user, you will want to be able to reference the user's content that needs work, and offer helpful steps to improving that content.
- If your platform has help pages, or a help-themed forum, be sure to link to them.
- If editing content isn't necessarily easy or straightforward, you may want to offer guidance on this as well. Many woes of new users are caused by taking the first steps towards learning how to use a new platform.
- Link to examples of good content that are related to what the user is trying to convey or express. We tend to emulate "good content" after we learn what "good content" is.
This is a very personal and one-on-one way of mentoring a new community member. Some communities have it so ingrained in their culture that they have templates for this sort of thing... Wikipedia is a good example: if you've ever made a first edit, it will likely have been followed up by a templatized welcome message by an established user.
You may need to be available to discuss or continue a line of communication with this user -- they may have additional questions. If you can't be available to do this, don't be afraid to invite other community members to welcome and mentor the user.