I have to agree with most here. It doesn't matter whether or not this would 'qualify' as poly or not. If the guy is talking about getting rid of you, he should be out of the picture. Your gf should get that.
I'm not opposed to people having one night stands. Would her having a one night stand with this guy count as her having a poylamorous relationship with him? No. Poly people can just have sex, too.