I firmly believe if you make yourself appear to be intentionally vague or obstructive to the police, you have a much better chance of either talking yourself into being a suspect or inviting greater police scrutiny.
The police, at least in this particular scenario, are trying to do their actual job of preventing or incarcerating assholes who show up at their ex’s place of work to beat them and trash their office.
If I were a police officer, any answer other than “I was sitting at my desk and next thing I know, Lucy’s ex-boyfriend was in here shoving her and throwing shit around” would sound odd, if not suspicious.
If I were the employer whose responsibility is the safety of my employees, I might start to wonder why you were being so uncooperative.
I might have questions about how Lucy’s ex found his way into the building, given most offices have security. I might wonder if another employee let him in. And maybe through the investigation, it comes to light that you were chatting with Lucy’s ex at the last company barbeque for a long time. Maybe you barely remember the conversation about the Jets or want bands you like or whatever,
So now all of a sudden, you’re an uncooperative person of interest with a potential link to the suspect.
IMHO, better to be minimally factual and truthful with the police to get you disqualified as a “person of uselessness” asap. “I was here doing my work sitting next to so and so,heard this, saw that, then you guys showed up.”