i'm worried that she'll do to me what she's done to him??
That is possible. And you have to consider that possibilty. If she does it now, she can do it again in the future. You can never be 100% sure.
once the relationship (sex) starts to get a bit stale as it always does do you think she'll start another affair?
The same answer as above.
Are some polish men like this
You can find this kind of men in every country. In Poland too for sure. I have seen it myself.
can some of them be as pretentious and devious as uk girls?
There are all kinds of personalities in both UK and Poland. It is not difficult to find pretentious and devious girls in Poland as well.