In terms of Prince2, let us say that an individual can influence external supplier Business Case. can he be picked for Senior Supplier for a project
The scenario you are describing is an example of a commercial customer/supplier environment in PRINCE2.
In such a commercial context, there are (at least) 2 Business Cases: the customer's Business Case and the supplier's Busines Case.
(remark: if there's more than one supplier, there will be multiple supplier's Business Cases.)
One of the key decisions related the the project organization in this case has to do with who can take the role of the Senior Supplier. Will the other Project Board members have enough trust in a person from an external organization? What if sensitive issues have to be discussed, e.g. related to extra funding or changes, or even the termination of the contract with the supplier...
PRINCE2 offers 2 options in this case:
- appoint the external person to the Senior Supplier role. If sensitive discussions would come up, this person might be excluded from participating in those. (e.g. leave the meeting room for these topics)
- Appoint a person from the customer's side, that could be the liason. This could for instance be a procurement manager, or contracts manager.
The Project Board is accountable for the success (or failure) of the project, and needs to provide unified direction, i.e. it's important that they have a unified vision on what exactly that direction should be for the project.
It will be the Executive's call who will be appointed to the role of Senior Supplier, taking into account the level of trust he/she has in either one of the above options. Be aware that the Executive is the ultimate decsion maker! He/she needs to make sure that an effective Project Board is put in place.