I assume he has a code reader? Any mention of any fault codes being thrown?
Just to throw a few things into the mix off the top of my head:
- tdc sensor. Renault updated the design - the newer version is blue in colour, the older one is black. If you have a black one it might be worthwhile looking at that.
- possibly immobiliser/key related?
- worth checking engine bay loom wiring for any damage/chaffing and cleaning all connections. There are also a couple of big white connectors (one in the fuse box and one under the scuttle panel) that can cause problems due to oxidisation/corrosion. Check the ecu plug is firmly connected as that can loosen up. Check earth straps too - there is one from the gearbox to the chassis iirc.
- Clio 2s seem pretty sensitive to battery voltage anyway, even if the battery comes up ok on a test they can still cause problems - often the trip meter resetting is a sign that the battery is on the way out.