It really rather depends on the pitch of the roof and the type of tiles.
I guess that in so far as health and safety is concerned, walking up a tiled roof is a real no no.
Depending yet again on circumstances, you can often extend your ladder up along the the tiles, but you often need either someone footing the ladder for you, a nice soft lawn to plunge your ladder into or a convenient car you can use for a giant wedge
Obviously there is WFP, but I presume you do not have that, so it all depends on the individual account and the level of risk you are prepared to take.
If it is too dangerous then leave it and explain why to your customer.
Ian