Friend's boyfriend..

So I just spoke to my friend and she's asked me to post this for help. 
She used to be able to get a good 20-25 minutes out of her and her guys sexy time.. But suddenly, over the last few weeks it's turned into SECONDS.. 
He's in really good shape and their relationship is stable, but what could this be due to? Like, what are some causes of this and what can she do to help?