Most Helpful Girl
I think the best you can do is be there for your friend. Spend time with him, talk to him, listen to him, do things with him.
I don't think she is lying about the reason why she broke up. You say that she was acting weird and hence your friend believes that "losing the spark" cannot be the real reason. However, in my opinion, those things go hand in hand. She probably did lose interest and hence was acting weird for a while because breaking up is always hard, and she might have had an inner conflict for a while.
Like I say, be there for your friend, that is really all you can do at the moment. I am sure that once some time has passed your friend and his ex-girlfriend will be able to have a conversation about it, but right now the timing doesn't seem to be right.
Most Helpful Guy
If she's been in a relationship with him for at least 2 years, she probably won't have a reason (that she can think of), or even need a reason.
If your friend had been doing anything that she didn't like, she would have already told him. He could be a more perfect man than ever, and it's still likely to happen.
Read this, to find out why most women can't stay in love for long in a long term relationship.