As you can see no one has replied so far.
If you would like to know my opinion:
I think your sentence is okay, but I would add an "is".
Nothing is worse than when a guy starts hanging around his friend's girlfriend.
(It sounds more natural to me.)
**Neither a teacher nor a native speaker.**
Just ignore my post if you only want teachers' opinions, but don't expect it because you are the only one with special requests.
Student or Learner