If "grooming" him means that she's trying to get him used to the idea of a sexual/physical relationship, then I'm all for it. Is he married? Single? Married men are much more likely to engage in a sexual relationship, where a single guy might get confused about the idea and start thinking that she's looking for a new life partner. You might want to discuss the pitfalls of a single guy "falling in love" with her and the complications that could lead to.
As far as work relationships go, care must be taken when engaging with work colleagues. A sex partner at work treads a fine line. If he's mature enough to understand the dynamic and both of them approach it maturely, everything should be fine, but depending on each one's position as manager/subordinate, could lead to one, or both of them losing their job.