pimplumb21, Hi.
Interesting dilemma?
Two scenarios, as I see them?
1/. If an "end user" the householder, comes to your client and clearly states that [the client WANTS] a new Shower / WC / WHB and requests a cost to supply and install the three items, then in effect the house holder is seeking a cost for just that, no more.
2/. If the house holder asks your client to "design and install" a " Bathroom" then in this case it "could" be argued that a competent trades person would include a ventilation system, because a competent tradesman will be aware of the long term "implications"
Going back to 1/. above, at that time your client should advise the house owner under a "sort of Duty of care" that a ventilation system is advised. if your client submits his estimate in written form then a simple disclaimer at the foot of the estimate will exonerate your client from on-going grief?
Hope some of the above makes sense? it is a "back covering" exercise
Ken