First, there is NO correct time to "get over" your grief. The so called societal one year limit is preposterous. In former times and cultures,women obeyed the prevailing custom of wearing black until the first year was officially over. They withdrew from society and basically "died" along with the deceased. Now we have work mandates. Three days off for the death of a loved one and back to work again! Three days!!
We need to recognize that grief and loss need support, and comes in all forms. There is grief that no one recognizes, sometimes not even the bereaved. Examples are loss of a pet, a relationship perceived as abusive, loss of a gay partner, a disapproving parent finding out that their child is gay, a child with birth defects who you expected to be "perfect", loss of job, retirement, the empty nest syndrome...When a divorce occurs, usually the person leaving is perceived to be the "villain". Never mind what went on behind closed doors. All of these people need our support as much as do those who've lost someone to death.
So how can you support them? It's not like you can say "I'm sorry" or go to a memorial. Most likely they haven't even talked to you about these things. But you can ask things like, "so how's it going for you?" or "life hasn't turned out to be quite as expected, has it?". Often they will feel relieved, and open up to someone who finally gets it. If they are in denial, and angrily ask what you mean, just say "I know that I would feel sad if it happened to me (or if I were going through this situation, etc)".
After that there are phone calls, condolence cards, casseroles and the usual rituals of support. Just don't stop when it's clear to you that the person still needs the support. Also you can find a support group that they can relate to. But when at some point the person seems to be relying on you to get by in the world and you're beginning to feel a heaviness yourself, that's the time to refer to a professional. And it's ok to tell them the truth if they don't want to go. True friends tell the truth.