How long can someone wait to decide to press charges?

My friend and his ex have had a rough break up. They were never married but had a child together. One evening she showed up to pick their son back up and gave him some mixed messages about sex so he decided to make a move then when he did she said she didn't want to and she called the cops. When they arrived and cuffed him for questioning she then said she didn't want charges pressed and that she only called so he knew she didn't want that anymore. The police said they had to make a report anyways since they were called out but then a month later when he attempted to obtain a copy of the report there was no record of the incident and when he sat down with the officer who had responded to the call the officer asked why a report was needed and basically made it sound like it was never filed. So my question is, is he in the clear or could his ex still file charges later on?