Some folks have ripped on the ex BF. They are exactly wrong.
The ex is genius. As pointed out, he was likely getting pressure to marry the broad. He doesn't want to go down that road and was getting bored with nailing her. So, he and the woman agree to break up but remain friends for sake of the dog.
He gets to hang with his dog but not have to take it for walks constantly, vacuum the house daily due to hair, clean up poo.
He is back to dating this chick. Marriage pressure is gone. He likes her as a person but does not have to live with her. They go on hikes and have dinner dates. They buck enough that it is not boring. Since he is now the other man, the broad is more energetic since she is doing the bad boy.
With the woman out of his life full time, he doesn't have to worry about getting caught railing his clients. He is a personal trainer. He is nailing at least 3 and likely 7 of his clients.
The OP is the loser in this. The GF and ex BF are the winners. Young FBGs should be taking notes. Find a hot chick that rocks in the sack. Get a dog together. Reap the benefits after the break up.