On a strictly clinical level, a fetish is something that MUST be present/worn/done/etc in order to become aroused sexually.

On a more lay or street level, "fetish" is applied much more liberally, often used to describe anything pertaining to BDSM. So it may simply be a matter of talking about the same thing but using different definitions to discuss it between you and your friend

Being a science wonk, I prefer the former definition. To me, it sounds like the heels (or whatever else) are things that make you feel sexy, rather than something you strictly need in order to become aroused. It seems as though the physical response or emotional state you experience whilst wearing them is more linked to the knowledge (or belief, or what-have-you) that the heels make you look more attractive.

If, on the other hand, you could only get jiggy with it (so to speak) whilst you wore a chicken suit and your partner played Yankee Doodle Dandy on a kazoo and you were on a submarine... well, that would be a fetish, in my opinion.