How often is the BF/GF behind your break-up

Listening to my roommate talk about his friend he had since college, they recently stopped hanging out, which might be harder cause they are co-workers at the same firm straight out of law school. 20 plus years of friendship, but his pal recently started dating someone and it became serious, serious to the point that they spend less and less time together, even working.

They way they are it just sort of seemed odd and I think most people would want to spend time with their romantic interest but my roommate is sure his pals new gf is behind all of this.

I personally never had this problem cause I always made a point to keep those things separate but for normal well adjusted people is this a norm?