I think the issue here is not how many times the keyword (key-phrase) appears, but how often it appears in context with everything else. If the page only has 50 words and 30 are the keyword, then yes, that could be cause for alarm, but I will assume that the word is used in context and reads well.
The site is a forum and as such, for the most part, content is out of your control. Google understands this but also index's based upon the content. This means SEO'ing a forum is very difficult naturally and results (Google position) is likely to fluctuate until the thread "dies" which ironically means it becomes more stable and will more likely retain a constant position (again, more factors such as how topical it is will also have an affect).
For example, the SE sites are full of the concern you mention. For example, a question may read "How do I do something with THIS", and normally the answers are similar too "You can use THIS ..." or "I've used THIS in a..." "When using THIS you need..." etc. As you can see, the keyword (THIS) is used often (and the more answers, the time it's used) but this is not considering keyword stuffing as it's all within context... For example, at the time of writing this post, the term "keyword stuffing" occurs 13 times on this page and there are only a total of 3 posts! I don't think it's going to be an issue