Hillstream loaches (not plecs) come from fast running streams; they need well oxygenated water. So this is why it may have died during a heatwave; hot water carries less oxygen. Ideal for cool tanks (22C is fine), not overstocked and with good aeration.
If your friend's house is centrally heated, adam, that may well be the temperature of his goldfish tank; after all, most people don't put cooling packs in their coldwater tanks, so they end up roughly the temperature of the surrounding air.