Well, it would be difficult for the employer to know that there is a problem unless the person in question is prepared to say something to them - they don't have to say much, but they just might like to mention that there is an issue that MAY affect their ability to perform to their best.
But some people are very private and reluctant to talk about their personal problems at work (especially to their employer) regardless of whether they feel 'safe' to tell their employer or not. In that case, if someone notices the person to be uncharacteristically quiet, for instance, that person should ask him/her if everything's OK. It might just open up a communication path that would eventually encourage the person from saying that he/she is having a problem.
I think mutual 'looking out for each other' and 'support' is important in this type of situation.