I think most senior consultants or architects do that work, but usually not only that work. I might call that specific role a "technical partner," or "vendor relations."
It's a good sign that your friend is thinking about bringing on a partner to help with the search process, as it can be very difficult to navigate all the parties' different incentives.
The one thing I would warn about is: be careful not to choose an agency just because their business development people make you feel heard. Often you only interact with their sales or business development people, and it's very difficult to evaluate an agency based on that. Agencies can have amazing business development people and dysfunctional development teams, or vice versa. It can be very tricky to navigate.
For clients that don't have as much experience working with agencies or other vendors, they usually look for larger agencies even though they charge more. Those agencies typically have dedicated business development people that serve two roles:
There's an uncomfortable aspect of this dynamic where many of the consultants who would be able to serve as a technical partner for this search process are also trying to sell you on having them do the implementation work as well. So you have to be really careful about misaligned incentives.
For example, a React shop might look at your needs and say you should overhaul your whole system to use Gatsby and React Native, regardless of your current tech stack. That's where their experience and your needs intersect, so that's what they feel comfortable offering.
A larger agency might not push a specific technical solution as hard since they have broader expertise, but that will also be reflected in the higher quote they'll give you for the implementation work. They might even be motivated to convince you to postpone any architectural decisions so that you need a larger agency like them.
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