Hi Paul,
Welcome your friend to the rather small band of us who collect modern French postage stamps (as opposed to the Classics, or postal history).
I agree with Mick Bister. Yes, Richard Broadhurst is my main contact for modern material and if you are in the packet circuit he often includes stamps and mini-sheets etc in the books he includes: and I occasionally I and others add some of our own.
Finding good used copies is increasingly difficult and is not helped by the disparity in pricing between the Y&T and SG catalogues. For example Y&T list the very attractive 2021 self-adhesive set of "Art in Chess" at 14 Euros whilst SG list the same set at £45.00. This, I think, reinforces the point made by Richard in a recent article that in France very few collectors appear to want used copies of stamps, so the SG pricing is perhaps a better indicator of scarcity. This would suggest that buying in France might be a cheaper option, but finding the material is harder. Delcampe occasionally lists some modern stamps.
best wishes
roger