I would get fibrin, complete the three days of heparin treatment recommended and have it again within a month. Almost without fail for the 15 months I was on PD. I started PD in early November, and the only time I had peritonitis was Christmas eve that same year, so I really don't think the peritonitis was caused by fibrin, or I would have had infections a whole lot more often.
The way my nurse explained it to me is that fibrin is just something that's always there, in everyone. We just get to be aware of it because of PD. Because of that, I don't think there's a diet to prevent it. Bottom line is that fibrin doesn't seem to be a problem in the long-run.