Do not overthink this. It is important to have a solid answer, but your interviewer won't be looking to call you out - as long as you are sincere, you will be fine.
From every interviewer I have ever spoken to (including in my capacity as a junior interviewer at Bain), everyone agrees that the people element is generally the most powerful. Therefore, your example of friends that work at BCG and enjoy it is perfect. If possible, I would add some slightly more concrete examples of why he/she enjoys it, and why that resonates with you.
If you have had the chance to meet BCG people at recruiting events at your University, I would also mention this, and the impact it had on your perception of the firm.
Finally, if there's a particular sector you are interested in that you know the BCG office you are applying to is particularly strong in, this is something worth mentioning.
What I wouldn't mention or place particular emphasis on is the generalist model, or that you don't have expertise in other areas. Most global consulting firms use a generalist model, so this doesn't convince me as to "why BCG". Further, the way you frame it makes it seem like a weakness that you do not have expertise in other areas, which is generally not desirable.
Honestly, do not over worry about this. The "why office/company" question is critical only when you are applying to an office where you do not have a clear link to (e.g. an American applying to Dubai office), or if you are making a major career switch and need to justify why (e.g. switching between consulting firms).