I totally agree with Roryrory.
I personally would never let a 2 year old out of sight in a house that is not baby-proofed, anyway. I mean, there are other things that are very dangerous for toddlers, not just pools. Cleaning supplies, nick-nacks, toilet brushes, etc. not to mention the damage a 2yo can cause in a very short space of time...
So I'd still go to her house but just make sure that I watch my child closely at all times.
Yes, pools are dangerous and they should be fenced, but I personally think that making an anonimous report is going a bit too far, no offence. IF you really want her to install a fence, then talk to her, tell her your reasons and let her know that you don't feel safe there and won't bring your child until she has a fence.
JMHO