a relative has had a garden makeover involving a rendered retaining
wall (2 - 3ft) behind which are flowers/shrubs etc in soil level to
the top of the retaining wall.

Looked fine when unpainted and new, then a bit mouldy so he cleaned
then painted the wall with exterior masonry paint - 1 year later the
paint is flaking off and he is not impressed.

What should he do to enable the paint to stick properly next time?

I am assuming that damp from the flowerbeds etc is the main culprit?

Cheers
JimK