I've seen progress tear up "friendships." It usually consumes the smaller guilds who have 2-3 great players that eventually feel constrained by the rest and then ditch them for a bigger more progressive one.
The only guilds I've been in where friendship mattered more than progress is where the people have met and hung out but this isn't always the case.
Good players will also tend to stick with one another as well because they have fun pwning content so it is possible for the two concepts to coexist.
Thankfully I'm in a great guild now. It's fun to be in and pretty damn good. Progress isn't everything. Sometimes just a fun mess around run of content is more enjoyable.
When i used to be a rl i always put progress in front of friendship.
Did that cause some arguments ? Yes, it just means that those *friends* werent really friends in the first place.
But that always depends on what your objectives are.
When i was rl i wanted my guild ( the guild i was officer in ) to aim as high as possible, and withme being one of the top players it could have been done but.. too many scrubs so i let it go and just went on with friend playing .. friends that i still talk nowdays .