'That's so fetch' is a quote from the 2004 film Mean Girls — character Gretchen Wieners tries to make 'fetch' (meaning cool) happen as a slang word, but Regina George shuts her down ('Stop trying to make fetch happen — it's not going to happen!'). The quote is now used self-consciously to celebrate something, especially in nostalgic Y2K contexts. If your teen says it, they are quoting a recognised film. Harmless and warm.