I got a boner from looking at my friend's feet

Its perfectly normal and not a fetish (which would have been if it was something extreme and you were obsessive about it and wanted to act on your urges). Straight men loves women's legs which are one of their most attractive features. My girl friend has really nice feet too and I love rubbing them. If you really, like the girl tell her you like her, and if you just want to stay friends, that's perfectly normal too, goodluck...